Ed's AV Handbook.com
Home Theater & High Fidelity Stereo Audio



Chapter Four
Video Reproduction

Page 8

Batting practice for the audio/video pro and a primer for the novice 



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Brightness, Luminance, Color
& more buzzwords

          Brightness is to video as loudness is to audio.  Bright is good.  Correct brightness is better. Excessive brightness distorts color and creates glare.  A better television image is created by an accurate reproduction of the image's brightness.  Luminance is its directional intensity.  TV brightness or luminace is measured in lumen, foot lamberts, and nits.  The choice of measurement depends on the display type and use.

Lumen
          The lumen is to luminance as the decibel is to loudness.  The peak lumen is a popular home theater measurement of brightness that measures a projected centered test pattern of
a white block filling 10% to 20% of the screen.  The remainder of the screen is black.

          The peak lumen measurement is limited to a small angle of radiation. Because of this limitation, the American National Standards Institute offers a more useful and stringent measurement, the ANSI lumen.
         
The ANSI lumen specification averages several measurements taken at different positions on a projected screen.  As a reference; a projector that produces 1200 peak lumen,
may only measure 200 ANSI lumen.


Foot Lambert

          Home theater projector/big screen enthusiasts are more concerned about the level of light reflected back towards the viewer than the brightness of a projector.  Projector brightness is important.  But t
he brightness of the reflected light decreases as screen square footage increases.  It also decreases as the distance between the projector and screen increases.  Therefore the ANSI lumen is not a sufficient unit of measurement.
          The foot-Lambert solves the problem.  The foot-Lambert factors a projector's brightness in ANSI lumen, the square footage of the screen, the projector's distance from the screen, and screen gain.  I
t is the home theater standard of screen brightness.
          As a reference, the Society of Motion Picture and Television Engineers recommends
a level of 12 to 22 ft L with a goal of 16 ft-L in a dark room.  16 foot-Lamberts is the reference of a commercial cinema.   If a low level of ambient light is present and unavoidable then 30 to 50 ft-L is the recommended goal.



Handbook Note:
 It is difficult to find reliable useful home theater projector/screen brightness data. 
Projector Central.com's
website calculator offers reliable estimates of foot-lambert image brightness
that discount inflated manufacturer projector specifications and factors lamp life.


Nit
          The NIT is the measure of television or video monitor screen brightness.  For reference; NTSC TV monitors were capable of about 100 Nits.  New HDR capable UltraHDTVs are capable of about 4000 to 10,000 Nits.

         
It's A Matter of Scale
          A better television image is created by an accurate scale of brightness; a scale that extends from the darkest black to brightest correct white.  This is described by the image's contrast ratio, gray scale, and color temperature.


Contrast Ratio
 

          Contrast ratio is an abused and often misunderstood television specification.  There are two types of contrast ratio measurement, dynamic and static, which has led to even further misunderstanding.

Dynamic

         Dynamic contrast ratio is the most quoted by manufacturers, magazines, and retailers. It refers to the deepest black versus the brightest white that a television/monitor can display; but not at the same time.  It is an “all off” versus “all on” test.  This simple test can generate impressive ratios such as  10,000:1,  20,000:1,  even higher.
(July 2017 - 160,00:1 contrast ratio reported per CEPro story .... What?)

A Contrasting Story

          A friend of mine shared the following story that put dynamic contrast into a real world frame of reference.
            He was attending an Imaging Science Foundation seminar.   During the lecture, he observed that a colleague of the lecturer had entered the rear of the room.   The lecturer acknowledged his colleague with a nod as he continued to speak until he finished his point.
            The lecturer then looked to his colleague and asked, “How did you do?”  His colleague answered, “300 to 1”.  The lecturer responded, “No way!”   His colleague insisted, “300 to 1”.  
Again the lecturer responded, “No way"; then paused and then said,  “Not unless.......... you turned off ....... the exit lights”.   His colleague admitted with a grin, “Yes”.

          The lecturer then turned to the seminar audience to explain their exchange.  His colleague had just completed the calibration of a commercial digital theater projector.  
300 to 1 referred to the contrast ratio he had achieved with the exit lights off.  The lecturer then said, “Given what you just heard, you should be asking; where do contrast ratios exceeding 1000:1 come from?”

          He explained that manufacturers often begin with the measurement of light in an absolutely dark room.  Well, there isn't any light.  Its a dark room.  They then turn on their projector with the lamp gain set at its highest adjustment and measure the light at the projection screen.  The difference is their contrast ratio specification.  The seminar audience responded with chorus of groans.

ANSI Contrast Static Ratio 
          The A
merican National Standards Institute contrast ratio specification uses a standardized checkerboard test pattern of black and white rectangles.  The ANSI standard simultaneously measures the difference between the darkest black and brightest white displayed side by side on a projected screen.  That is the source of the 300:1 number the ISF tech referred to.


Gray Scale
          The gray scale is a measurement of a displays transition from black through gray to white.  It is the most important TV attribute.  This measurement is to video as frequency response is to audio.  The gray scale measures the accuracy of luminance which in turn determines the amplitude of the red, green, and blue voltages.  
 
Color Temperature
          Have you ever heard the expression “red hot” or “white hot”?   They have their origins
in black smithing and welding.  As iron is heated with a welding torch; it changes in color from black to red, to white, to blue.   This is the basis for the concept of color temperature in degrees Kelvin.



C.I.E. Color
     
          T
his illustration is the
International Commission on Illumination official chart of visible color
establish in 1930.
          Envision the illustration as a three dimensional cone.  The X,Y points display the red, green, blue color mix points.  D65 (6500 Kelvin) is gray scale amplitude Z point for the color white.
          All color is specified in terms of its temperature.  Higher temperatures
>5,000K = cooler (blue to white) color;  lower 2,700 – 3,000K = warmer' yellow thru red colors.


Color Space
is the total specified space of human vision under the Color Chart cone.

Color Gamut is the color space allocated to a video specification such as HDTV.

Color Volume measures color gamut as a percentage of the cone space specified.  For example; UltraHDTV covers 75.8% of the total space.  HDTV covers 35.9% of the color space.

Bit Depth or Color Depth is the number of computer bits allocated to create a video color sub-pixel.  The number of bits determines the possible range of color shades.

For example; 8-Bit color provides up to 255 shades per red/green/blue sub-pixel for a total of 16.78 million colors.  10-Bit color provides up to 1024 shades for a total of 1.07 billion colors. 12-Bit color as employed by Dolby Vision provides even more shades.

 x.v.Color is the color space specification for HDTV.  It offers a palate of color that nearly doubles the area of the original NTSC color chart.

Deep Color describes 10 bit or more color depth.  For example; 10 Bit HDR10 and 12Bit Dolby Vision offer deep color depth.  Many UltraHDTV's can be enabled to reproduce Deep Color.

DCI P3 is the Digital Cinema Initiative spec that defines the color gamut of commercial digital cinema which covers 53.6% of the CIE Color Chart.  

Gamma is a fixed gray-scale luminance correction to accommodate human perception.  
If you're an old audio pro, gamma is similar to Fletcher-Munson loudness correction.


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Ed's AV Handbook.com
Batting Practice for the AV Pro and a Primer for the Novice.
Copyright 2007 Txu1-598-288   Revised 2018