Fig. 1:  Contrast sensitivity functions derived for 2-degree sinusoidal gratings at a luminance of 45 cd/m2 for red (625 nm), green (525 nm), and blue (475 nm) LEDs. Fig. 2:  Contrast sensitivity functions derived for 2-degree sinusoidal gratings at typical luminance values (red: 125 cd/m2, green: 35 cd/m2, blue: 5 cd/m2) for red (625 nm), green (525 nm), and blue (475 nm) LEDs. Fig. 3:  Contrast sensitivity functions derived for 10-degree sinusoidal gratings at a luminance of 45 cd/m2 for red (625 nm), green (525 nm), and blue (475 nm) LEDs. Fig. 4:  Contrast sensitivity functions derived for 10-degree sinusoidal gratings at typical luminance values (red: 125 cd/m2, green: 35 cd/m2, blue: 5 cd/m2) for red (625 nm), green (525 nm), and blue (475 nm) LEDs. TECHNICAL PAPER: Determining Contrast Sensitivity Functions for Monochromatic Light Emitted by High-Brightness LEDs (2004)

Determining Contrast Sensitivity Functions for LEDs (2004)

Light-emitting diode technology is quickly becoming the choice for many lighting applications that require monochromatic light. However, one potential problem with LED-based lighting systems is uneven luminance patterns. A uniform luminance distribution is important in some applications, such as backlighted signage. The question is: How much uniformity is required for these applications?

Presently, there is no accepted metric that quantifies luminance uniformity. A recent study proposed a method based on digital image analysis to quantify the beam quality of reflectorized halogen lamps. But to be able to employ such a technique to colored beams generated by LED systems, it necessary to have corresponding contrast sensitivity functions (CSFs), which describe the contrast sensitivity of the visual system as a function of spatial frequencies. CSFs do not exist for red, green, and blue light produced by high-brightness LEDs at visual fields and luminances typically found in backlighted signage. Therefore, the objective of this study was to develop a family of CSFs for colored high-brightness LEDs, specifically at 2-degree and 10-degree visual fields.

EXPERIMENT

Two human factors experiments were designed: one, to replicate an existing CSF for white light sources to use as a baseline for comparison, and a second to determine the CSFs for monochromatic light produced by red, green, and blue LEDs at 2-degree and 10-degree visual field sizes.

The experimental apparatus consisted of a light integrating box with sine wave grating patterns mounted at the back. The gratings varied in contrast and spatial frequency, and could be switched out during the experiment. Depending on the experimental requirements, the patterns were illuminated by either red, green, or blue LEDs or two 40-watt incandescent lamps. Subjects viewed the illuminated sine wave gratings from 9.5 feet away for the 2-degree visual field tests and from 1.8 feet away for the 10-degree visual field tests. The experiments were conducted at an equal luminance level (45 cd/m²) and at varied luminance levels typically found in backlighted signage (125 cd/m² for red, 35 cd/m² for green, 5 cd/m² for blue).

CONCLUSIONS

• 2-degree visual field and equal luminance level of 45 cd/m²: The CSFs for red, green, and blue illumination overlap in the higher spatial frequency region but separate out slightly at lower spatial frequencies. (Fig. 1)
• 2-degree visual field and varied luminance levels: Peak values of the CSF increase with increasing luminance levels and shift to higher spatial frequency values. (Fig. 2)
• 10-degree visual field and all luminance levels: The CSFs of all three colors overlap. Neither luminance nor color variation seem to have an impact on the CSFs. Peak values of the CSFs occur at smaller spatial frequencies compared with the 2-degree field, and the values of contrast sensitivity for the 10-degree case are greater at all spatial frequencies compared with the 2-degree case. (Fig. 3 and Fig. 4)