Photopic And Scotopic Light

The human eye is a light sensing system with a retina (the photoreceptive medium) and an aperture (the pupil). The retina is composed of two types of photoreceptors: cones and rods. The cones provide for color vision and viewing fine detail while rods are associated with night vision.

Why is this important?

This is important because cones and rods have different spectral responses. Cones are most receptive to green light at the wavelength of 555 nm while rods are most receptive to blue-green light at 507 nm. So, if you introduce a light source that takes advantage of these subtle differences, you can greatly improve visual performance in the work environment.

Light meters and photometric devices that measure light output are generally calibrated to the cone spectral sensitivity known as the photopic response. As a result, the light output of a lamp (lumens) is rated only in terms of its photopic content. Historically, the rod spectral sensitivity, known as the scotopic response, has generally not been considered relevant for interior lighting. However, many lighting professionals now believe that these photopic devices do not accurately reflect our perceptions of lit environments, and photopic lumens do not tell the whole story of a lamp1s effectiveness.

Researchers have found that pupil size decreases when light levels are increased. But increasing light levels isn1t the only way to shrink pupil size. In typical building interiors, it is scotopic rather than more in the blue-green will be more efficient in contracting the pupil than white light that is deficient in blue-green composition.

So, what1s the relationship between pupil size, scotopic light and blue-green spectra?

Nearly all people have imperfections in the lens of the eye. These imperfections cause optical aberrations (i.e. aberrant rays are allowed to reach the retina and these aberrant rays cause blurred vision). A smaller pupil size reduces these aberrant rays and visual performance is generally improved. A lamp that produces the blue-green light can maximize the scotopic response, shrink the pupil size and improve a person1s brightness perception and their visual performance (acuity, contrast sensitivity and depth of field).

While smaller pupil size can be achieved by simply raising photopic lumen levels, the more energy efficient way of reducing pupil size is to provide lighting that has a color spectrum which is more tuned to the pupil1s response. JAFtech high-bays can accomplish this. Even though JAFtech's SkyBay has fewer photopic lumens than a standard MH HID lamp, its stronger scotopic light negates the photopic deficiency.

With the smaller pupil, visual performance is maintained or improved at a lower lumen level. This can offer substantial energy savings. For tasks that require higher visual acuity, it therefore follows that light sources with this blue-green spectral distribution are more efficient at producing visually effective light.

How can we compare lamps from the scotopic versus photopic perspective?

Lamps are currently rated solely by using the photopic function as a calibration factor. This gives us the photopic lumens found in lamp makers1 catalogs. To include the scotopic sensitivity function in lamp evaluation, a modifying factor is applied to the traditional lumen. A ratio of scotopic to photopic lumens (S/P) for a given spectral power distribution (SPD) provides a simple mathematical means to incorporate the sensitivities of both the rods and cones.