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Ed's AV Handbook
Batting Practice for the AV Professional
and primer for the novice

Chapter 4  Page 8

Video Reproduction
Brightness, Luminance, Color

Incadescent Light Bulb

Brightness

Brightness is to video as subjective loudness is to audio.  Much as the ear to sound, the eye is not equally sensitive to the entire bandwidth of light. 
The human eye is very sensitive to the wavelength of green light, less sensitive to red light, and even less to blue light.

Luminance

Luminance is the objective directional intensity of light.  Luminance is to video as audio amplitude or volume is to audio. 
And as the audio decibel, the lumen, foot-lambert, and nit are measures of luminance.  The choice of measurement depends on the type of display used. 

The Lumen & Peak Lumen 
& The Projector

The lumen is to luminance as the decibel is to sound.  The peak lumen is a measure of video projector luminance. 
It measures a projected centered test pattern of a white block filling 10% to 20% on the projection screen.  The balance of the screen remains black.

ANSI Lumen 
& The Projector

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

Foot Lambert  
& The Projector & Screen

Home theater projector-big-screen enthusiasts are more concerned about the level of light at the viewer than at the screen.  Projector luminance is useful. 
But reflected luminance decreases as screen square footage increases.  Luminance also decreases as the distance between the projector, screen, and viewer increase.  The ANSI lumen does not sufficiently facilitate the home theater enthusiast's objective.

The Foot-Lambert (ft-L) offers a more useful practical measurement.  The Foot-Lambert is a factor of projector ANSI lumen, screen square footage, projector to screen distance, plus screen gain.  As a reference, the Society of Motion Picture and Television Engineers (SMPTE) recommends a level of 12 to 22 Foot-Lamberts with a goal of 16 ft-L in a dark room.  If a low level of ambient light is unavoidable, then 30 to 50 ft-L is the recommended goal.

Foot Lambert Illustration

Foot-Lambert computation is challenging.  Reliable home theater projector/screen luminance data is scarce.  However, Projector Central.com offers a dependable website calculator.  Enter the projector brand, model #, screen size, screen gain, plus projector to screen distance.  The calculator then produces a reliable estimate of the projection system's luminance in foot-Lamberts.

The Nit
& TV Screen

Nit TV View

The Nit is a measure of luminance from a direct view television screen.  For reference, NTSC TV studio monitors were capable of about 100 Nits.  High definition and UltraHD televisions are capable of about 400 to 2,000 Nits.   One Nit is equal to approximately 3.426 projector ANSI Lumens.  

Select this LINK for additional ANSI Lumen to Nit information.

It's a Matter of Scale, Grayscale.

Accurate luminance reproduction precisely ascends, scales, from absolute black to specified white light.  An accurate scale, or grayscale, generates accurate color as accurate audio frequency response generates accurate musical timbre. 

Grayscale

Contrast Ratio

Contrast ratio is the difference between the brightest and darkest screen image.  There are two types of contrast ratio measurement -- dynamic and ANSI static.  The contrast difference establishes the range of luminance plus shades of color.

Dynamic Contrast

The dynamic contrast ratio is the most quoted by manufacturers, e-zines, and magazines.  Dynamic contrast refers to the deepest black versus the brightness white that a television, studio monitor, projector/screen can display, but not at the same time.  Dynamic contrast is an all-on versus all-off test.  This simple test can generate impressive numbers such as 20,000:1 or higher.

ANSI Contrast Screen

ANSI Static Contrast Ratio

The ANSI static contrast ratio 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 projection screen.

A Contrasting Story

A friend shared the following story to put contrast measurement into a real-world frame of reference. 
He had attended an Imaging Science Seminar.  During the presentation, 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.  He then paused and said, "Not unless ... you turned off the exit lights".  Yes, his colleague admitted with a grin.  300 to 1 was his contrast result.

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 ANSI static 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 more than 10000 to 1 come from?"

He explained that many projector manufacturers begin with a measurement of light in an absolutely-dark room.  Well, there isn't any light. Then they turn their projector on with the lamp gain set to its highest adjustment.  Then they measure the light at the projection screen. This non-ANSI difference is their contrast ratio specification.  The seminar attendees then responded with a chorus of groans.  The point, be skeptical of contrast ratio claims.

Color Temperature

Have you ever heard the expression 'red hot' or 'white-hot'?  Both have their origin in blacksmithing and welding.  As a welding torch heats a piece of iron, it changes from black to red to white, to blue.  This effect is the basis for the concept of color temperature in degrees Kelvin.  6500° Kelvin is the video color temperature standard of white light.  Film uses a white standard of 5500° Kelvin.

Color Temperature

C.I.E. Color Chart

The C.I.E. is the international organization of color specification.  The range of visible color, or color space, is the x, y, z, points of the C.I.E. Color Chart. 
It resembles a three dimensional cone -- narrow at the top -- wide at the base.

The chart's x and y points stake the color points of red, green, and blue.  All color is more or less bounded within the area of the red, green, blue points. 
The chart's z-axis, marks the amplitude, or luminance, of the color. 

Some refer to the z-axis as brightness.  But that's not correct.  The z-axis is luminance, the objective measurement of light.  Brightness is the subjective-response of the eye.  The D65 point, near the top, equals 6500° Kelvin, the color white.  The z-axis lowest point equals black.  Points from black to D65 track the gray-scale
CIE Color Chart

Color Space

Color space is the total specified space of human vision outlined in the C.I.E. color chart.

Color Gamut

Color gamut is the area within the color space allocated to a video specification such as HDTV.
HDTV = 35% of the color space.  UltraHD = 76% of the color space.

Color Gamut

Color Volume

Color volume measures the percent of the color gamut achieved by the video display.

x.v. Color

x.v.Color is the HDTV color gamut specification.

DCI P3

DCI P3 is the Digital Cinema Initiative (movie theater) color gamut specification.

Gamma

Gamma is a fixed grayscale correction to accommodate the subjective response (brightness) of the human eye. 
If you're a seasoned AV pro, gamma is similar to the Fletcher-Munson audio loudness curve.

Gamma Chart
Gamma Photo

EOTF 

Electro-optical-transfer-function is a dynamic grayscale correction, not fixed, to accommodate the subjective response of the human eye.
EOTF continually optimizes night/dark and day/bright images on a frame-by-frame basis.

Bit Depth or Color Depth

Color depth is the number of computer bits allocated to create video color.

Bit Depth

Deep Color

Deep color describes 10-bit or more color depth. 
For example HDR10+ offers 10-bit color, Dolby Vision offers 12-bit color.
The increase from 10-bit to 12-bit extends the color palette from one billion to four billion colors.

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Ed's AV Handbook   
Copyright 2007 Txu1-598-288 Revised 2021


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