INSTRUCTIONS: [Currently this program requires a Cambridge Research
Systems photometer, but you could easily adapt it to use another
photometer.] Plug your photometer’s USB cable into your computer,
carefully place your photometer stably against your computer’s screen,
set PARAMETERS (below), then run. The results (including the best-fitting
n-bit-precision model) will be displayed as a graph in a MATLAB figure
window, and also saved in three files (in the same folder as this file)
with filename extensions: png, fig, and mat. The filename describes the
testing conditions, e.g.
EXPLANATION: Using Psychtoolbox SCREEN imaging, measures how precisely we
can control display luminance. Loads identity into the Color Lookup Table
(CLUT) and measures the luminance produced by each value loaded into a
large identical patch of image pixels. (This program varies only the
luminance, not hue, always varying the three RGB channels together, but
the conclusion about bits of precision per channel almost certainly
applies to general-purpose presentation of arbitrary RGB colors.) The
attained precision will be achieved mostly by the digital-to-analog
converter and, perhaps, partly through dither by the video driver. Since
the 1980’s most digital computer displays allocate 8 bits per color
channel (R, G, B). In the past few years, some displays now accept 10 or
more bits for each channel and pass that through from the pixel in memory
through the color lookup table (CLUT) to the digital to analog converter
that controls light output. In 2016-2017, Mario Kleiner enhanced The
Psychtoolbox SCREEN function to allow specification of each color
component (R G B) as a floating point number, where 0 is black and 1 is
maximum output, so that your software, without change, will drive any
display and benefit from as much precision as the display hardward and
Typically you’ll run MeasureLuminancePrecision from the command line. It
will make all the requested measurements and plot the results, including
the best-fitting n-bit-precision model. Each figure is saved as both a
FIG and PNG file, and the data are saved as a MAT file. The data are also
returned as the output argument. It has luminance out “data.L” vs
floating point color value “data.v”.
To use this program to measure the precision of your computer display you
need three things:
- MATLAB or Octave. http://mathworks.com
- The Psychtoolbox, free from http://psychtoolbox.org.
- A Cambridge Research Systems photometer or colorimeter.
It’s plug and play, taking power through its USB cable. You could easily
modify this program to work with any other photometer.
As of April 2017, Apple documents (below) indicate that two currently
available macOS computers attain 10-bit precision from pixel to display
(in each of the three RGB channels): the Mac Pro and the iMac 27” retina
desktop. From my testing, I add the Apple’s high-end MacBook Pro laptop
(Retina, 15-inch, Mid 2015). I tested my MacBook Pro (Retina, 15-inch,
Mid 2015) and iMac (Retina 5K, 27-inch, Late 2014). Both use AMD drivers.
Using MeasureLuminancePrecision, I have documented 11-bit luminance
precision on both of these displays, enabling both o.use10Bits and