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Measuring Contrast Ratio

The easiest way to judge the success of your display design is how it looks to the eyeball of the average observer. Unfortunately, how it looks to your eyeball is somewhat subjective, and some objective data, using simple equipment, is probably necessary.

The detection of a given display character depends on the luminance and chrominance of the character compared to the luminance and chrominance of its background.  If the parameters of the character are the same as its background, the character will be indistinguishable.

The luminance contrast C may be defined as as the ratio of the difference between character and background luminance DL, to the luminance of the character. That is:

  C = Lmax - Lmin / Lmax
  C = DL / LB ( For LED's or negative image LCD's)
  C = DL / LO ( For positive image LCD's)
Where LO and LB are the luminance of the segment and the background respectively.

The value of contrast is always less than or equal to 1. The higher the value of contrast, the brighter and more legible the character will be. In the case of either LED's or LCD's, the more usual measurement is the contrast ratio, CR. Assuming that we are driving our display into saturation, (we're turning it all the way on), the contrast ratio is usually defined as:

  CR = Lmax / Lmin
  CR = Ls / Lo ( For LED's or negative image LCD's)
  CR = Lo / L ( For positive image LCD's)

or finally,  

C = 1 - 1 / CR

In the case of direct drive LCD's, when the applied voltage is sufficient to fully activate the segment, the highest contrast ratio is obtained. The absolute minimum contrast ratio is 2:1, preferably >5:1, and  for actual production displays, about 200:1 for a reflective display should be the minimum target.  For a transmissive display, about 100:1 should be the minimum target.

For multiplexed displays, the segments never go into saturation, and therefore the above contrast ratios are difficult to attain. The contrast ratio is sometimes measured at V50, where V50 is the voltage at which the contrast reaches 50%. For multiplexed displays even 50% contrast is rarely attained.

Next month we will investigate how luminance changes the apparent contrast ratio of any display. It will be obvious that for low light level applications, both contrast and luminance must be considered for a satisfactory design.

I hope this helps with your display designs. As usual, please call us at (440) 232-8590 and talk to one of our applications specialists if you have any questions about this or any other