Today's compact yet powerful front
video projectors may leave you with a too bright an image to
watch! This being especially so if you intend to watch a
projected image in a darkened room – which after all, is
most often the case with a home theater setup.
Defining ‘Screen Gain’
Prior to proceeding with our discussion,
it is important that one has a full understanding of what the
term ‘screen gain’ is all about.
It may sound strange that we talk about
‘gain’ when in reality a projector screen is nothing more
than a passive device. Yet there is - it is all a question of
how the screen surface distributes the reflected light.
Screen gain is a measurement of
the reflectivity of the screen surface. It measures the
ability of the screen to direct incident light back to the
audience.
The gain number represents the ratio
between incident light and reflected light. A flat matte white
surface has a gain of approximately 1. A gray screen has a
gain less than 1 and therefore, it attenuates incident light.
Reflective screen surfaces have gains greater than 1; these
projection screens direct more incident light back to the
audience.
Gain is always measured in front and
perpendicular to the screen at the point of maximum
brightness; this represents peak screen gain and occurs at the
zero viewing angle perpendicular to the screen surface.
For a screen with gain greater than one,
the gain drops as one moves to the side and view the screen at
an angle to the perpendicular drawn from the center of the
screen. The angle at which screen gain falls to half the peak
is referred to as ‘half gain viewing angle’. A
person viewing the screen from this angle will see the image
half as bright as the person seated at the center. The more
one moves further away from the center, the dimmer the
projected image will be.
The higher the peak screen gain, the
narrower the supported viewing angle. This is the price one
has to pay for a higher screen gain - the higher projector
screen gain at the center is in fact achieved by directing
more light towards the center viewing positions rather than
allowing for a uniform reflected light over a wider angle of
view.
Projected Image Brightness:
Matching the projection screen gain
with the ambient light and the video projector
output is essential if you want to ensure that you will
end up with a correct level of projected image brightness.
This is an extremely important issue. Do
not forget that a basic requirement for a great movie
experience is a comfortable environment. This means that the
time one spends watching a movie should be as comfortable as
possible. Exposure to a bright projected image is
uncomfortable on the eyes - even when this is for just a few
minutes, least imagine when watching a two-hour movie.
A Tricky Matching Process!
Ambient Light Conditions, Image
Brightness & Screen Gain
A projected image is at its best when
viewed in total darkness - this helps take away any visual
distractions - rendering it easier for the viewer to get
deeper immersed into the movie action.
In the home, achieving a completely
darkened room is almost impossible except in the case of the
dedicated home theater setup. The amount of ambient light
present has an impact on the resultant projected image
contrast. This calls for the need to increase the image
brightness to maintain the correct level of image contrast.
The tricky issue is to arrive at the
correct level of image brightness for a pre-set level of
ambient light. This in view that projected image brightness is
a function not only of the projector output, but also of the
screen gain, and image size.
The explanation that follows should
help you get a better understanding of the relationship that
exists between these three inter-related parameters.
Display Brightness and Screen Gain
The brightness output level of
your projector is a measure of the projector output power in
terms of light intensity. Please keep in mind however that the
resultant projected image brightness is a different issue.
Image brightness falls in proportion to the area of the
projected image size - the bigger the projected image is, the
dimmer it will look for a fixed level of projector brightness.
The projected image brightness -
more specifically the Luminance level for a projected
image, is a measure of the light reflected from the projector
screen area. It is measured in foot-Lamberts, and is
defined as:
ANSI-lumens of your projector divided
by the square footage of Screen.
(Note that one foot-Lambert is equal
to 1 ANSI lumen per square foot.)
To arrive at the actual level of light
reflected from the screen surface, the above result must be
multiplied by the projector screen gain.
A useful reference here is what has been
defined as 'sufficient brightness' by the Society of
Motion Pictures and Television Engineers, also known as
SMPTE.
In standard 196M, the SMPTE determined
sufficient brightness for showing motion pictures in a
darkened room using a unity gain matte white projector screen,
as 12 - 22 foot-Lamberts. In practice, the luminance target
level is set to about 16 foot-Lamberts, while the average
bright scene brightness level in a movie theater is typically
60% to 75% of this target value.
Display brightness is in itself an
entirely subjective term; what’s more, it is all relative to
the amount of ambient light falling on the screen surface.
The following rules of thumb would
surely come to 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. One may argue that TV is a totally different
technology – this is true, but this target level for
image brightness or luminance, still holds good for most
circumstances.
How does all this translate in
practical terms?
If one is viewing an image in typical
very low ambient light conditions (say less than 2
foot-candles - equivalent to approximately 22 Lux), then the
minimum required light level ‘illuminating’ the
screen surface should be around 10 foot-candles to achieve
sufficient image contrast.
For a matte white projector screen
surface with a screen gain of one, this translates to a
minimum of 10 foot-Lamberts 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.
Similarly, if viewing were to take place
under normal ambient light room conditions (typically 10
foot-candles or 110 Lux), applying our first rule of thumb to
achieve the required contrast level would result in a
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.
Note:
- 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).
Now, let’s consider as an example 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
It is clear that our 1000 ANSI-Lumens
projector would produce a too bright an image when the latter
is viewed in a darkened room, even when using a standard matte
white projector screen with unity gain. Similarly, use of this
projector in conjunction with a projector screen surface
having a gain of 2 may result in a too bright an image even
when viewing takes place under normal ambient light
conditions.
