| Understanding Digital Imaging Technology |
Understanding digital projector markets
Topic status automatically displays here - do not remove.
Bookmark this topic
Print this topicLet's consider the projector market. There are two distinct categories:
-
workplace (data/presentations); and
-
home (movies/entertainment).
The major differences between these market categories can be summarized into a features list:
| Projector Feature | Work (data/presentations) |
Home (movies/entertainment) |
|---|---|---|
| Aspect ratio | 4:3 | 16:9 |
| Resolution |
640x480 (VGA, NTSC) 720x540 (DVD) 768v576 (PAL) 800x600 (SVGA) 960x720 (STD) 1024x768 (XGA) 1600x1200 (UXGA) |
854x480 (WVGA, NTSC WIDE, 480p) 1024x576 (WSVGA, PAL WIDE, 576i ) 1280x720 (WXGA, 720p/i) 1366x768 (768p/i) 1920x1080 (1080p/i) |
| Brightness (luminance) | ||
| Contrast | ||
| Colour (range and accuracy) | ||
| Weight (portability) | ||
| Size (dimensions) | ||
| Style (appearance) | ||
Brightness
About brightness, luminance and illuminance"Brightness" is a relative term with many meanings and interpretationsmostly subjectivewhich makes its use in accurate determination of the light output of a light source (such as a projection device) that much more difficult. So as to avoid confusion with other uses of the word, "brightness" should never be used for quantitative description, but only for non-quantitative references to physiological sensations (using the human eye) and perceptions of light (colour, temperature, energy). The proper term for describing the amount of light emitting from a light source is called "luminance", and for light emitting display devices which you directly look at such as Cathode Ray Tube (CRT) TV's and monitors, Plasma displays, or Liquid Crystal Display (LCD) monitors and TV's, the light output measurement is made in candelas per square metre (cd/m2) commonly referred to as "nits", and is made using a luminance meter. The light being emitted from a projector is not looked at directly, but rather looked at indirectly on a projection screen or surface, so cannot be measured directly in nits, as that is not how the image is viewed. The light being reflected (with a front projector) or transmitted (with a rear projector) from the screen surface is what you see, so that is what has to be measured, as that is a more accurately measure of the relative "brightness" of what you're looking at. The proper term for describing the amount of light emitting from a lit surface is called "illuminance", and the light measurement is made in lumens per square metre (lm/m2) commonly referred to as "lux", and is made using an illuminance meter. However, the amount of light on the surface of the screen is also effected by other light sources such as the ambient light level in the immediate vicinity caused by daylight, overhead lighting, background lighting, computer monitors, and similar light sources. The amount of light reflected by the screen is dependant upon the screen colour, angle, and surface material, as well as its distance from the projected light source. Any and all of these variable factors contribute or detract from the screen surface light level. To overcome most of these variables, the American National Standards Institute (ANSI) produced a standard for determining the projector "brightness" commonly known as the "ANSI lumen". |
Projector brightness was measured in 'ANSI lumens' (replaced by 'IEC lumens' in 2003) being the average of nine measured light readings across the reflection of a projected white image from a white screen. To provide a basis from which to relate the ANSI Lumen projector lighting levels to other lighting levels, take a moment to consider the luminance and illuminance tables below of typical lighting levels for sources and activities:
| Luminance (cd/m2) |
Light source |
|---|---|
| 100,000 | direct sunlight |
| 4000 | clear blue sky |
| 2000 | cloudy cover |
| 300 | full moon |
| 120 to 150 | 100W electric lamp |
| 13 | single wax candle flame |
| 0.0005 | starlit sky |
Luminance is the amount of light being emitted by a light source measured in cd/m2.
| Illuminance (lx) |
Suitability for particular activities and places |
|---|---|
| 1600 to 2400 | close inspection of minute detail |
| 800 to 1600 | inspection of fine detail, retail outlets |
| 600 | office desk activities, detailed reading, writing |
| 300 | notice boards, displays, counters, canteens, kitchens, casual reading |
| 200 | reception areas, entries, dispatch, bulk stores, living rooms |
| 100 | open storage, loading bays, washrooms, general areas, dining |
| 50 | corridors with heavy traffic, passageways around machinery, stairs, restrooms |
| 20 | corridors with light traffic, good outdoor area lighting |
| 5 to 10 | outdoor pathways, undercover carparks and driveways |
| 0.1 to 5 | poor outdoor area lighting, driveways |
Illuminance is the amount of light on a surface measured in lux (lm/m2).
The effectiveness of shining (or projecting) a light onto a surface (like a projection screen) to create a visible image (as projectors do) is directly effected by other (extraneous) light in the immediate vicinity, which may also shine onto the surface and detract from the quality of the projection (by so-called 'washing it out' or making it difficult to see). Less extraneous light means that the intended brightness, contrast and colour of the image are more likely to be accurately reproduced in the projection.
Therefore, rooms that are purposely darkened (like commercial cinemas), do not require an overly bright projection because extraneous light is restricted and controlled. Likewise, business and home theatres which can control the ambient lighting levels in the room, can utilize the better image quality produced by low lighting level projectors (designed for use in controlled lighting environments).
Alternatively, a room with a brighter ambient lighting level will necessarily require a brighter projection so that the projected image can be clearly seen in amongst the other extraneous light. The brighter lighting level in the projection will directly affect the contrast and the colour accuracy reproduction of the projection. Mostly, these affects are negative, when compared with a low lighting level projector.
In a brightly lit room, the image clarity (colour and contrast) of a projected image is greatly diminished. The brighter colours and especially the whites will splash light into the surround areas causing a white-wash effect of the image clarity (washing out the colour), degrading the overall projected image quality. This can be prevented by careful placement and siting of the projector and screen, directed and controlled lighting within the room, well fitting window coverings, and the selection of an appropriate brightness projector to suit the particular installation situation.
Brighter projected images require colour enhancement to counteract the brightness whitewashing, so must be artificially enhanced before being projected. This is easy enough when processing a digital image, but is not an exact science, so when done well, won't be seen, but when anything less than well, will be noticeable and add to the further degradation of the image quality.
Brighter projected images require a higher contrast level, as the darker shades become swamped by all the extra light. This results in a so called raised 'black level', where everything which was previously near dark, becomes relatively dark, and difficult to distinguish from black.
Definitions
- ANSI Lumen
- The standard of measuring the average brightness level of a display device such as a projector onto a flat projection screen. The ANSI lumen measurement represents the average lighting lumen value calculated across 9 grid locations over a flat projected screen image, as opposed to the standard lumen value measured from the curved concave inner surface of a sphere being equal in lumen value because all parts of the surface are equidistant from the central light source point.
- The American National Standards Institute (ANSI) was subsequently replaced in July 2003 by the equivalent International Electrotechnical Commission (IEC) lumen standard 61947-1. To determine the ANSI/IEC lumen level, the total screen area is divided into 9 equal grid areas and the illuminance at the centre of each area is measured in lux with an illuminance meter. The average lux reading is found by adding the nine readings and dividing the total by nine. The lux value (lumens per square metre) is converted to lumens by dividing the average value by the number of square metres of the entire projection. The result is in ANSI lumens.
- For example, lets say that on an 2.5m (98 inch) diameter 4:3 aspect screen with a display area of 1.732m2, we measured and calculated an average illuminance level of 100 lux. If we divide the average lux value (100lx) by the screen display area (1.732m2) the result is (100/1.732=) 57.7 ANSI lumens. Similarly, if the average lux value was higher, say at 1000lx, then the resultant ANSI lumen value would be correspondingly higher at 577 ANSI Lumens.
- Brightness
- To avoid confusion with other uses of the word, "Brightness" should never be used for quantitative description, but only for non-quantitative references to physiological sensations and perceptions of light. Brightness (illuminance) in terms of the amount of light emitting from a projector is measured in ANSI lumens, and from a Plasma display or computer monitor is measured in nits (cd/m2). The projected image can never be "brighter" than the source. The apparent brightness of a source when you look directly at it must not be confused with its luminous emission (flux). For example, a nearby source of light may appear much brighter than a faraway source with much higher luminous flux. This is because not all of the emitted flux is received by your eye, and you receive less flux from sources that are farther away. At the threshold of vision the dark adapted observer can see a flash if it contains on average 90 photons at the cornea or 9 at the retina. This is equivalent to observing the light from a candle 30 miles away on a clear night.
- Candela
- The SI unit of measure of luminous intensity of a light source; representing the light emitted in any one particular direction from one candle, expressed as 'cd'.
- Lumen
- The SI unit of measure of luminous flux; the rate of flow of light emitted by a point light source, or received on a surface, having a uniform intensity of one candela. Symbol lm. Equal to the amount of light given out through a solid angle (of 1 steradian) by a point source of one candela intensely radiating equally in all directions. The steradian is the cone of light spreading out from the source which would illuminate one square meter of the inner surface of a sphere of 1 m radius around the source. If a light source emits one candela of luminous intensity into a solid angle of one steradian, the total luminous flux emitted into that solid angle is one lumen. One lumen is defined as the luminous flux of light produced by a light source that emits one candela of luminous intensity over a solid angle of one steradian. A wax candle generates 13 lumens; a 100 watt electric tungsten filament bulb generates 1,200 to 1700 lumens.
- Luminous flux
- The measure of the perceived power of light; the rate of flow of energy of visible radiation (light) emitted in all directions by a point light source, adjusted to represent the sensitivity of the human eye. The luminous flux accounts for the sensitivity of the eye by weighting the power at each wavelength with the luminosity function, which represents the eye's response to different wavelengths. The luminous flux is a weighted sum of the power at all wavelengths in the visible band. The unit is the lumen (lm).
- Luminous intensity
- The luminous flux (lumens) emitted by a point source of a solid angle cone of light in a given direction, measured in candela (cd). Also referred to as candlepower. Luminous intensity should not be confused with luminous flux, which is the total perceived power emitted in all directions. Luminous intensity is the perceived power per unit solid angle.
- Lux
- The SI unit of measure of illuminance, defined as an illumination of one lumen per square metre (lm/m2). Symbol lx.
- Illuminance
- The luminous flux (lumens) measured on a surface per unit area in lux (lx). Symbol E. 1 lx = 1 lm/m2. Illuminance was formerly often called brightness, but this leads to confusion with other uses of the word.
- Luminance (nits)
- The luminous intensity (candela) per unit projected area of a surface; the amount of light that passes through or is emitted from a particular area, and falls within a given solid angle. e.g. if a very small portion of a surface has a luminous intensity of 1cd (candela) in a particular direction, and if the orthogonal projection (on a plane perpendicular to the given direction) of that portion has an area A, the luminance in that direction is 1/A candelas per unit area. The unit is candela per square metre (cd/m2). Symbol L = Light. Also called luminosity. One candela per square metre is commonly called a nit.
- Other (non SI) units of luminance are the footlambert (fL), the candela per square inch (cd/in2) and the apostilb (asb). To convert to cd/m2, multiply:
- fL x 3.426 cd/m2
- cd/in2 x 1550 cd/m2
- cd/ft2 x 10.76 cd/m2
- asb x 0.3183 cd/m2
- electro-optical.com
- 1 lambert = 3183 cd/m2
- 1 footcandle = 1 lm/ft2
- 1 footcandle = 10.76 lm/m2 (lx)
- Some optical definitions sourced from a glossary of color science: http://tigger.uic.edu/~hilbert/Glossary.html
- Achromatic
- A neutral color such as black, white or grey (see chromatic).
- Brightness
- Attribute of a visual sensation according to which an area appears to emit more or less light. (CIE 45-25-210.) The perceived amount of light coming from an area. "Brightness" is often restricted to apply only to lights and "lightness" is used for the corresponding dimension of the colors of surfaces. One of the three standard elements of color appearance (the other two are hue and saturation). Its colorimetric equivalent is luminance.
- Chroma
- Attribute of a visual sensation which permits a judgment to be made of the amounts of pure chromatic color present, irrespective of the amount of achromatic color. (CIE 45-25-235.) For colors of the same hue and brightness, chroma and saturation are equivalent. Chroma will increase with brightness, however, even if saturation is held constant. Chroma is used instead of saturation in the Munsell color system.
- Chromatic
- A hue. All colors other than the neutral colors white, black, and the pure greys, are chromatic. The word "color" in ordinary language is often used to refer exclusively to chromatic colors, e.g., color vs. black-and-white television.
- Chromaticity
- The hue and saturation of a color ignoring brightness.
- Chromaticity diagram
- A diagram that represents the unit plane (the plane defined by the equation X+Y+Z=1) in a tristimulus space. The location of a stimulus with a particular set of tristimulus values on a chromaticity diagram represents its direction from the origin of the space ignoring its distance. The chromaticity diagram is often used as a convenient approximation to a constant brightness plane in the tristimulus space.
- Farnsworth-Munsell 100-hue test
- A standard test for deficiencies of color vision in which the subject is asked to arrange a set of 100 colored chips in circle. Subjects with normal color vision will arrange the chips in a specific order with very few deviations. Subjects with abnormal color vision will deviate from the normal arrangement in ways that provide information about the nature of their defect.
- Fluorescence
- The absorption of light at one wavelength and its re-emission at a longer wavelength. Fluorescence plays an important role in the perceived color of many objects: the unnatural brightness of "day-glo" paints is due to fluorescence.
- Hue
- Attribute of visual sensation which has given rise to color names such as: blue, green, yellow, red, purple, etc. (CIE 45-25-215.) Hue differences depend primarily on variations in the wavelength of light reaching the eye. One of the three standard elements of color appearance (the other two are brightness and saturation). Its colorimetric equivalent is dominant wavelength.
- Luminance
- The intensity (power weighted by the overallspectral sensitivity of the eye) per unit area of a light source.
- Munsell color system
- A widely used system for describing the color appearance of samples. The Munsell system uses matching against a set of samples and interpolation between them to arrive at a designator for the appearance of a given test sample. Color appearance in the Munsell system is characterized using sets of three symbols, for example 2.5 YR 5/10. 2.5 YR is the hue, 5/ the value (= lightness), and /10 the chroma.
- Nanometer
- One billionth (10-9) of a meter. The most common unit used for characterizing the wavelength of light in visual science.
- Saturation
- Attribute of a visual sensation which permits a judgment to be made of the proportion of pure chromatic color in the total sensation. (CIE 45-25-225.) Pink and red differ in saturation with the red being the more saturated. The spectral colors are all maximally saturated examples of their hues and differ in this respect from pastels which are desaturated. One of the three standard elements of color appearance (the other two are hue and brightness). Its colorimetric equivalent is purity.
- Spectrum or visible spectrum
- Band of electromagnetic radiation ranging from wavelengths of approximately 400 to approximately 700 nanometers, corresponding to the sensitivity of the human eye. Sensitivity does not drop to zero at the standard endpoints of the visible spectrum, but is so low that light outside these limits rarely has a significant effect on visual response.
- Cones
- One of the two main classes of photoreceptor found in the vertebrate eye. Cones produce usable outputs only at relatively high light levels and provide the main inputs for color vision. There are three types of cones in the human eye, each type having a different spectral sensitivity.
- Rods
- One of the two main classes of photoreceptor found in the vertebrate eye. Rods are very sensitive to light but fail to produce a usable signal at high light levels. They mediate night vision and have little effect on color vision in daylight.
- Retina
- The inside layer of the back of the eye that contains the photoreceptors and associated neurons. The earliest stages of visual processing take place in the neurons of the retina.
- Photoreceptor
- A light sensitive neuron. Photoreceptors interact with light which produces changes in their electrical properties which are communicated to other neurons. They constitute the first stage in the physiological process which underlies vision. The human retina, like the retina of most vertebrates, contains two broad classes of photo-receptors, rods and cones.
- Spectral sensivitiy
- The sensitivity of a photoreceptor at each wavelength in the visible spectrum. For a given wavelength, this is equal to the ratio of the photoreceptor response at that wavelength to its response at the most sensitive wavelength. Different photoreceptor types have different spectral sensitivities.
http://www.practical-home-theater-guide.com/projector-screens.html
1 foot-Lambert = 1 ANSI lumen/ft2
projected image brightness = ANSI lumens of your screen/square footage of screen
It is important to keep in mind here that display brightness is in itself an entirely subjective term - it is all relative to the amount of ambient light falling on the projector screen surface.
The following rules of thumb would surely come to some assistance in this respect:
The contrast ratio between the projected image and the ambient light level falling on the screen should be at least 5:1.
This is necessary for the eye to perceive a real impression of brightness. If this contrast ratio is not achieved, the projected image will not be considered of adequate brightness level.
When viewing takes place under normal ambient light conditions, the luminance level should be close to 50 foot-Lamberts.
This level of luminance is derived from the fact that SMPTE indicated a target level of image brightness for a CRT TV as 50 foot-Lamberts, this in view that a TV is normally viewed under normal ambient light. It is true that TV is a totally different technology, but this target level for image brightness or luminance, still holds good for most circumstances.
Thus, if one is viewing an image in typical low ambient light conditions (say under 2 foot-candles - equivalent to approximately 22 Lux), applying the first rule of thumb would imply that the minimum required image brightness illuminating the screen surface should be around 10 foot-candles.
For a matte white projector screen surface with a screen gain of one, this would translate to a minimum of 10 foot-Lamberts (approx. 110 Lux) in terms of light reflected from the screen for image brightness. This is close to the SMPTE typical requirement for average luminance when viewing pictures in a darkened room.
On the other hand, if viewing were to take place under normal ambient light room conditions (typically 8 - 10 foot-candles or 85 - 110 Lux), applying our first rule of thumb to achieve the required contrast level would result in a required luminance level of 50 foot-Lamberts (approx. 540 Lux) for the same matte white projector screen surface. This is also in line with the target luminance level detailed in rule 2 for viewing under normal ambient light conditions.
———————————————————————————————————————— Light illuminating a surface is measured in lux (Lx = lumens/m2), or foot-candles.
Light reflected from an area (luminance) is measured in candelas/m2 (cd/m2) or foot-lamberts (fL).
To help you get a better understanding of the relationship between these different parameters, and in particular, the need to choose a projector screen with the appropriate screen gain, lets consider what will happen with different projector screen gains for say a given 1000 ANSI-Lumens projector and a 100" diagonal screen size.
| Projector Screen Gain | 1 | 1.5 | 2 |
| Image Luminance in foot Lamberts | 30 | 45 | 60 |
From the above figures, it is clear that our 1000 ANSI-Lumens projector is too bright to view images in total darkness even when using a standard matte white projector screen with a gain of one.
Similarly, use of this projector in conjunction with a projector screen having a screen gain of 2, may result in a too bright an image even when viewing takes place under normal ambient light conditions.
The above goes to show that if you want to enjoy the best results from your video projector screen set-up, you cannot proceed with a projector screen purchase in isolation from your projector; nor it is possible to purchase a video projector and a projector screen without taking into account the ambient conditions in your home theater.
http://www.answers.com
A unit of measurement of the amount of brightness that comes from a light source. The standard lumen rating of a data projector is the average of photometer readings at several points on a full white image on the screen.
Technically, lumens measure "luminous flux." A wax candle generates 13 lumens; a 100 watt bulb generates 1,200. The lumen rating is a critical specification when choosing a data projector. In a darkened room, 500 lumens may be ample; however, in a conference room with normal lighting, 1,000 lumens would be better. In a room with bright daylight, 2,000 lumens is preferred.
An ANSI lumen rating uses an average of several measurements taken across the face of the light source. A small room typically requires from 200 to 300 ANSI lumens, whereas a large room may require from 400 to 600. A large auditorium may need 2000 or more.
http://www.evertech.net/Sales/Projection/ProjectionTerms.asp
Appropriate ANSI lumen rating depends on room lighting and screen size. As a general rule of thumb, a rating of 400 to 600 ANSI lumens is adequate with a 60" diagonal screen with room lights off, but you should look for something in the 700 - 1000 range for a 100" to 150" screen with dim lighting, and at least 1300 ANSI lumens when you go to a 300" screen or want to project in bright lighting conditions.
.http://www.projectorpeople.com/tutorials/bp_buyersguide.asp
Portable Projectors: Ultra-portable projectors (range from around 1000-2500 lumens in brightness. Low or lights-off presentations are generally considered less desirable, as they allow for less visual interaction with the audience. A projector with at least 800 lumens is desirable for use in rooms where there will be little ambient light. You should also consider the size of your typical audience when selecting the right brightness. Larger audiences require larger images, which also require higher lumen output.
1000-1200 lumens for lights-off, low ambient light, presentations 1500-2000 lumens with some ambient light 2000-2500 lumens with bright ambient light Training/Conference Room Projectors Typically, the brightest projectors (around 2500-7000 lumens) are slightly less portable. These larger projectors (around 10-25 lbs.) are perfect for conference rooms, training rooms and classrooms.
2500 lumens for audiences of less than a hundred with ambient light 3000 lumens for audiences of 100-200 with ambient light 5000 lumens for audiences of 100 or more under bright lights
Projectors typically fall within 2 ranges of brightness: below 2500 lumens and above 2500 lumens.
Controlled lighting-If you plan to use your projector in
a room where there are no windows, such as a basement, or if you use your projector
primarily at night, any of today's projectors will provide a bright image.
Low ambient light- Little to no light entering room.
Some ambient light- Some additional light in room.
Slightly dimmed, window blinds leaking some light.
Bright ambient light- Windows open during daylight
hours, lights that cannot be dimmed like in an open office settings. Bright enough
for audience note taking.
*For video signals, your actual lumen output may
vary from manufacturers specified levels, due to the lower luminance output from
video signals. Variations differ from projector to projector, so contact a Projector
People representative to find out more.
|
|
When selecting from the huge range of
projectors available, you might be wondering if brighter is always
better. You usually end up paying more for higher brightness, so it's
worth checking out if you really need it. http://www.projectorpoint.co.uk/projectorbrightnessadvice.htm Some brightness basics
Brightness is measured in ANSI lumens.
Trivial fact #1: ANSI is the American
National Standards Institute, who came up with a way of measuring
projected brightness. Unfortunately, though, the nice people at ANSI
left a little room for 'scientific interpretation' when this standard
was launched. Hence a 1000 lumen projector from one manufacturer isn't
always as bright as a 1000 lumen projector from another, so don't
take these measurements too literally.
Trivial fact #2: This is one reason why in the early days of Powerpoint presentations so many people used yellow text on a blue background. Projectors weren't too bright, so it made sense to use two bright colours with good contrast against each other.
How to choose the brightness that's right for you
Now we've done the basics, here's how to apply them to choosing the
right amount of lumens for you. There are three factors you need to
consider:
Footnote: Some pundits would say that audience size is a factor to consider. We've left it out, partly to upset the pundits but also because we assume you'll use a screen that's big enough for the guys at the back to see your content. |
Reference
AS1680 1976 Australian Standard Code of Practice for INTERIOR LIGHTING AND THE VISUAL ENVIRONMENT
www.npl.co.uk/publications/light/spectrum.html
www.electro-optical.com
www.answers.com
www.coleprojectors.com
See Also
Terrestrial TV channel frequencies for Australia
and New Zealand | 
Lotech Solutions' Tips, Tricks, and Procedures
email webmasterCopyright Lotech Solutions. All rights reserved.