How do you measure color contrast?

In a previous blog post on Color Contrast, we introduced how color contrast principles from the US Access Board are being applied to fitness equipment guidelines to improve accessibility for people with vision impairments.

Our primary question was, ‘How do we measure color contrast?’  Potential methodology was drafted, reviewed, and revised multiple times by experts in various fields relating to the project.  As a result, Beneficial Designs developed a draft method to measure color contrast by using spot-meter technology that is used in the photography industry.  This protocol was recently tested by 7 subjects in the Beneficial Designs’ Laboratory.

Subjects individually made measurements on three pairs of colors on a Standard Color Chart

  • One clearly below 70% contrast
    #17 Magenta was compared to #20 Purple
  • One clearly above 70% contrast
    #2 Light Skin was compared to #10 Purple
  • One very close to 70% contrast
    #16 Yellow / #8 PurplishBlue
Figure 1. Color card with markers for test colors

Figure 1. Color card with markers for test colors

Each of the 7 subjects:

  • Measured the ambient light level at the “signage”
  • Verified that ambient light level is above the minimum requirement
  • Measured the contrast in each color pair using spot meter
  • Made each measurement at a three foot distance
  • Made each measurement on a one-inch radius target
  • Repeated each measurement 3 times
Figure 2. Subject measuring color contrast using a spot meter

Figure 2. Subject measuring color contrast using a spot meter

Each of the 3 measurements were averaged and the color contrast percentage for each color pair was calculated using Equation 1:

Contrast = [(B1 – B2)/B1] x 100                                                    Equation 1

where B1 = light reflectance value (LRV) of the lighter area and
B2 = light reflectance value (LRV) of the darker area.

The color contrast percentages were then compared across subjects. Preliminary data suggests that the calculated color contrast values are repeatable (see Table 1). The protocol will be refined based on test results. The protocol and repeatability results will be presented at the November 2011 ASTM meeting.

Table 1. Color contrast repeatability results across 7 subjects

Color Pair: Average: Standard Deviation

#17 Magenta v #20 Purple : 64%: 0.01
#16 Yellow v #8 PurplishBlue: 73%: 0.01
#2 Light Skin v #10 Purple: 76%: 0.01