Waveform 04 Bias Lighting
Posted: Wed Jun 15, 2005 1:17 pm
Originally published 10/01/2004, HDTV Magazine, editor Dale Cripps
By Richard Fisher
Bias lighting has many applications and all revolve around creating a specific perception of your image by controlling your perception of color, or how much the iris in your eye moves. To a degree this can be accomplished with standard lighting but all light has a color to it and for video D65 is preferred.
The most common use of bias lighting in professional circles revolves around creating a specific ambient light color temperature and level behind the display in concert with a specific wall color in the viewer's field of vision. The purpose is to keep your perception of color at a reference point, D65, which is the same color temperature as the display. DVE and Sound And Vision both have a section devoted to explaining this further providing great visual examples of how what is really white becomes another color due to the other color in your field of vision around the display.
Another application of bias lighting is to create a specific perception of black. Black does not exist as a color or palette in light reproduction and is simply expressed and created by the absence of light. It is an optical illusion. If your display does not do blacks very well, meaning after you have calibrated the contrast and brightness you still find light emanating from your screen in a darkened room, then a bias light can easily resolve this problem. The purpose here is to create a ceiling for how much the iris in your eye will open up. The bias light prevents it from opening up greater than the amount of light from your display, making it impossible for you to see the light still emanating from a black screen, thus creating the illusion of jet black. This illusion is easily demonstrated using a calibration DVD and going to the image on the disc for setting black. Turn on the lights in the room and set your black level. Now turn them off and you will see that what was black is not. Now set the black level again under these conditions and turn the lights on. Now you can hardly see anything and your blacks will be buried.
Viewing fatigue is the final application for bias lighting. This is caused by the iris in your eye constantly being in motion as it reacts to the light output of your display. Another source is when your viewing distance creates a situation where the image occupies too small an area on the surface of the rods and cones in the back of the eyeball compared to the light level around that image. Both are caused by a display with very high light output. High output displays do not work well in dark viewing environments. Even if the image is occupying 30 degrees of your vision the difference between absolute black and peak white may be far too great.
The first step is to confirm that the contrast is set properly for the display technology. If you can still get an accurate response with the contrast turned down then do so. During calibration I will put up a 20IRE window and 100IRE window and switch back and forth. If the 100IRE window is too bright I can feel it right away. Bottom line is the 100IRE window should still be comfortable to view and when you switch to the 20IRE window it should be plain as day. If it takes a few moments for your eye to adjust then your iris is kicking in to compensate and ultimately when you go from a bright image to a dark image you will be blinded until the iris opens back up and vice a versa. If turning down the contrast is not an option then your only recourse is bias lighting. In this application the bias light output is set to 10% of your peak white and the black level is adjusted accordingly. If the image is not occupying enough of your field of vision you may find the fatigue has been resolved at the expense of color accuracy due to the wall color and decor in your room, as discussed last week.
KEEP IT SIMPLE
If you are not viewing your image at 3-4 screen heights in a darkened room then a bias light and attention to room decor and wall color is likely necessary to maintain color accuracy.
If your display has high light output causing viewing fatigue then a bias light can resolve this problem if you cannot accurately reduce the light output of the display.
If your display is not providing deep blacks then a bias light can resolve this problem.
LINKS
http://www.cinemaquestinc.com/ideal_lume.htm
If you have any feed back to this article please feel free to post your comments now by clicking on POST reply. All emails received concerning this article are open to being posted and published along with the article unless the sender requests otherwise.
Respect Our Copyright! DO NOT copy and paste this complete review or article to post or distribute. That is a violation of US copyright. Please feel free to quote small passages and please provide a link to the full review. Thank you!
By Richard Fisher
Bias lighting has many applications and all revolve around creating a specific perception of your image by controlling your perception of color, or how much the iris in your eye moves. To a degree this can be accomplished with standard lighting but all light has a color to it and for video D65 is preferred.
The most common use of bias lighting in professional circles revolves around creating a specific ambient light color temperature and level behind the display in concert with a specific wall color in the viewer's field of vision. The purpose is to keep your perception of color at a reference point, D65, which is the same color temperature as the display. DVE and Sound And Vision both have a section devoted to explaining this further providing great visual examples of how what is really white becomes another color due to the other color in your field of vision around the display.
Another application of bias lighting is to create a specific perception of black. Black does not exist as a color or palette in light reproduction and is simply expressed and created by the absence of light. It is an optical illusion. If your display does not do blacks very well, meaning after you have calibrated the contrast and brightness you still find light emanating from your screen in a darkened room, then a bias light can easily resolve this problem. The purpose here is to create a ceiling for how much the iris in your eye will open up. The bias light prevents it from opening up greater than the amount of light from your display, making it impossible for you to see the light still emanating from a black screen, thus creating the illusion of jet black. This illusion is easily demonstrated using a calibration DVD and going to the image on the disc for setting black. Turn on the lights in the room and set your black level. Now turn them off and you will see that what was black is not. Now set the black level again under these conditions and turn the lights on. Now you can hardly see anything and your blacks will be buried.
Viewing fatigue is the final application for bias lighting. This is caused by the iris in your eye constantly being in motion as it reacts to the light output of your display. Another source is when your viewing distance creates a situation where the image occupies too small an area on the surface of the rods and cones in the back of the eyeball compared to the light level around that image. Both are caused by a display with very high light output. High output displays do not work well in dark viewing environments. Even if the image is occupying 30 degrees of your vision the difference between absolute black and peak white may be far too great.
The first step is to confirm that the contrast is set properly for the display technology. If you can still get an accurate response with the contrast turned down then do so. During calibration I will put up a 20IRE window and 100IRE window and switch back and forth. If the 100IRE window is too bright I can feel it right away. Bottom line is the 100IRE window should still be comfortable to view and when you switch to the 20IRE window it should be plain as day. If it takes a few moments for your eye to adjust then your iris is kicking in to compensate and ultimately when you go from a bright image to a dark image you will be blinded until the iris opens back up and vice a versa. If turning down the contrast is not an option then your only recourse is bias lighting. In this application the bias light output is set to 10% of your peak white and the black level is adjusted accordingly. If the image is not occupying enough of your field of vision you may find the fatigue has been resolved at the expense of color accuracy due to the wall color and decor in your room, as discussed last week.
KEEP IT SIMPLE
If you are not viewing your image at 3-4 screen heights in a darkened room then a bias light and attention to room decor and wall color is likely necessary to maintain color accuracy.
If your display has high light output causing viewing fatigue then a bias light can resolve this problem if you cannot accurately reduce the light output of the display.
If your display is not providing deep blacks then a bias light can resolve this problem.
LINKS
http://www.cinemaquestinc.com/ideal_lume.htm
If you have any feed back to this article please feel free to post your comments now by clicking on POST reply. All emails received concerning this article are open to being posted and published along with the article unless the sender requests otherwise.
Respect Our Copyright! DO NOT copy and paste this complete review or article to post or distribute. That is a violation of US copyright. Please feel free to quote small passages and please provide a link to the full review. Thank you!