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#276778 - 10/30/09 02:13 AM Bias Light
ClubNeon Offline
connoisseur

Registered: 02/06/09
Posts: 3442
Loc: Western Maryland, USA
I've mentioned a few times that I have a "bias light" behind my screen, and that everyone else should too.

I've attempted to photograph how it affects the image, but because of what the bias light is actually doing, i.e. providing a reference for the eye, the camera is not fooled as easily.

A bias light does two major things: One, it provides a reference white (provided you have neutral colored walls). Human vision tend to compensate for the color of lighting. You'll still see an egg as being white even under a purely red light. A scene in a movie that's lit by a red light will have less visual impact as your eyes will adjust. A constant 6500K reference will make the colors on screen appear more true. Two, it keeps a base level of lighting in the room. Watching a screen in complete dark will cause a constant change in the pupil where a dark scene will make a following bright scene wash out. So some room light is required, but if the light is in front of the screen it can wash out the display, and highlight other objects near the display.


This first image is taken with the over-head room light on. All the bulbs in my apartment are 5900K, 5-phosphor, CFL, with a CRI of 93. They're very good bulbs, almost as good as the bias light. I white balanced on the wall behind the screen.


The second image is with the lights out. This time I went with an auto-white balance as that's what your eyes would be doing without any other reference.


This shot is lit by the bias light located behind the TV. Again the white balance is on the wall. This light is a florescent tube, with a color temp of 6500K, and a 7-phosphor coat, with a CRI of 96. This is a bulb used to color proof commercial goods in a viewing booth. Also notice how you can no longer notice the speaker or the PS3. The rest of the room is equally dark.

What I couldn't simulate is the slow reaction of the eye's iris to changing lighting conditions in a film. If you want a better idea of the subtle differences between the shots, open them in separate tabs and switch back and forth. Notice especially with the first and last that the walls appear to be the same neutral white, but the color on the display becomes more saturated with the bias light shot.

There's also something cozy about a soft glow emanating from behind the TV.
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#276779 - 10/30/09 02:20 AM Re: Bias Light [Re: ClubNeon]
jakewash Offline
shareholder in the making

Registered: 12/26/03
Posts: 10389
Loc: Calgary, Alberta
Intersting, thanks. \:\) I may have to do something like this for the new room.
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#276793 - 10/30/09 06:33 AM Re: Bias Light [Re: jakewash]
MarkSJohnson Offline
shareholder in the making

Registered: 09/27/04
Posts: 10666
Loc: Central NH
Ray3 was always a big fan of bias lights if I remember correctly....
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#276795 - 10/30/09 08:14 AM Re: Bias Light [Re: MarkSJohnson]
Ajax Offline
axiomite

Registered: 12/30/03
Posts: 6212
Loc: Cleveland, Ohio
I use an Ideal-Lume standard, 6500k bias light behind my TV. Wouldn't watch without it.
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#276796 - 10/30/09 08:25 AM Re: Bias Light [Re: Ajax]
terzaghi Offline
connoisseur

Registered: 04/04/07
Posts: 4840
Loc: Tulsa, Oklahoma
What TV is that?
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#276797 - 10/30/09 08:25 AM Re: Bias Light [Re: Ajax]
Murph Offline
axiomite

Registered: 10/05/06
Posts: 6612
Loc: PEI, Canada
Very interesting. I have never heard of this effect.

Is the type of light the bulb produces the key or the fact that it is located behind the TV the most important item or both?
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#276805 - 10/30/09 08:54 AM Re: Bias Light [Re: Murph]
CV Offline
Founder, Axiom Upgrade Club
shareholder in the making

Registered: 07/20/06
Posts: 11053
Loc: Richland, WA, USA
Also, is anyone doing this for projection screens, or is it only TVs?
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#276806 - 10/30/09 08:55 AM Re: Bias Light [Re: Murph]
ClubNeon Offline
connoisseur

Registered: 02/06/09
Posts: 3442
Loc: Western Maryland, USA
I use the Ideal-Lume Pro, but that's probably overkill. I had a Standard before, and really can't tell the difference.

The TV is a Pioneer PDP-5020FD, 50" plasma.

Placing any light behind the TV to give about a 15% gray value to the surround wall is enough to bias the eye's iris, and improve perceived contrast. The reference 6500K white with a very high color rendition index, biases the white balance of the eye to make the hues read more correctly.
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Pioneer VSX-1018AH-K, PDP-5020FD, DV-79AVi
Axiom M22s, VP150, QS8s
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#276807 - 10/30/09 08:57 AM Re: Bias Light [Re: CV]
ClubNeon Offline
connoisseur

Registered: 02/06/09
Posts: 3442
Loc: Western Maryland, USA
 Originally Posted By: CV
Also, is anyone doing this for projection screens, or is it only TVs?


Projection screens are big enough to fill the field of view that is it hard to make use of a border to bias the eye. I've also read that the larger the screen the less it is needed. But going from 32" to 50" didn't decrease the effect to me.
_________________________
Pioneer VSX-1018AH-K, PDP-5020FD, DV-79AVi
Axiom M22s, VP150, QS8s
Sony PS3, surround backs
-Chris

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#276810 - 10/30/09 09:33 AM Re: Bias Light [Re: jakewash]
Micah Offline
connoisseur

Registered: 11/16/08
Posts: 1789
Loc: Indiana you hoser!!!!
Wasn't there a Panasonic or Sony TV that came out not too long ago that radiated light behind the set? Only as I remember it, it wasn't a white light, it would radiate the main color on the display at the time, so it was constantly changing. In the commercial they touted the back light as making the whole experience more involving of the viewer. However it seems they missed the whole purpose of it if they were using colored lights instead of a constant white source like you are.

Very interesting indeed.
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