Gray-scale Expander


The gray scale expander is designed to increase the resolution available for low-contrast applications such as contrast threshold settings.

It is based on one of Denis Pelli's ideas and contains a passive- resistive network. The green input is attenuated by approximately a factor of 8 and the the blue is attenuated by a factor of 64.  The inputs are then summed and recombined.  The single output is then connected to the single BNC input of a monochrome monitor or "daisy-chained" to the red, green and blue inputs of a color monitor.  For the scheme to work without severe loss of luminance, the monitor must either be (a) a monochrome monitor or (b) an RGB monitor with a termination switch, which sets the monitor impedance extremely high.  Most monitors do not have this switch--we try to keep a current listing of monitors that do.

 In addition to the gray-scale expander box seen here, we offer an adapter kit consisting of a set of BNC cables, Y-connectors, 75-ohm terminator and a VGA-BNC cable to accomplish the daisy-chaining.

Some FAQs:

1.  What about the issue of color temperature constancy across luminance levels?

This is a potential problem whether or not you use a gray-scale expander.   However, there is some good news.   High-end monitors tend to have separate gain (contrast)  and cutoff (intensity) controls for each color gun.  We have found that careful iterative calibration with the use of a colorimeter or spectrometer can achieve quite good results.

2.  What if my monitor does not have the impedance termination switch?

Another approach is to connect the single output to a single color of the RGB monitor.  This will not yield an achromatic image of course, but it will work without loss of luminance (aside from the loss associated with not having all 3 color guns active).  In fact, this solution solves one potential problem of RGB monitors--the lack of color temperature constancy across luminance levels.  With this approach, a termination switch is not needed.  It is possible that this approach may work better for some monitors than others.

3.  What if my monitor does not have BNC inputs?

We can supply a cable that will go from the BNC output of this box to the more common HD15 VGA-style connector.  The chances are, however, that if a monitor does not have BNC connectors it will not have a termination switch either and you will need to use the single gun solution described above.

4.  How do I program with this device?

Obviously, since red, green and blue values are all used to determine a single monochrome value, some special programming is needed.  However, our linearization products automatically produce a table of RGB triplet values for each of 32, 768 equal-interval gray levels.  Our software development system has functions for reading the table and setting the appropriate look-up table (LUT) values on the graphics card.  Also, our applications come with generic linearization tables and are pre-programmed to use them.  All you need to do is to select the color mode from the menu and enter the desired contrast or luminance.