E7 Photometry and Vision Optics

Topic

Radiometry measures the physical energy carried by light at each wavelength—without distinguishing whether the human eye perceives that energy—and weights all wavelengths equally. However, the eye does not respond equally to all wavelengths; as the photoreception model describes, retinal photoreceptors absorb light differently depending on its wavelength. Consequently, two light sources with identical radiometric energy but different wavelengths can produce vastly different sensations of brightness in the eye. The photometric model corrects for this difference by weighting the radiometric energy at each wavelength according to the eye's specific spectral sensitivity at that wavelength. It assigns greater weight to wavelengths to which the eye is more sensitive and less weight to those perceived more weakly, thereby yielding a photometric quantity that represents the actual brightness perceived by the eye, rather than merely the physical energy carried by the light.

The spectral sensitivity used to weight radiometric measurements is not fixed; rather, it depends on which photoreceptors dominate vision at any given moment. When the eye is adapted to high light levels, cones dominate vision—as described by the photoreception model—and the model weights the radiometry based on the combined spectral sensitivity of the cones. Conversely, when the eye is adapted to very low light levels, rods dominate vision, and the model shifts to weighting the radiometry based on the spectral sensitivity of the rods—a sensitivity profile shifted toward wavelengths different from those dominant during cone-based vision. Thus, the same physical light source can be weighted differently—and ultimately perceived with a different level of brightness—depending on the eye's state of adaptation at the moment it receives the light. The amount of light—weighted by spectral sensitivity—that passes through the pupil and reaches a specific point on the retina constitutes retinal illuminance. It is this retinal illuminance—rather than the raw radiometric energy emitted by the light source—that determines the extent to which photoreceptors must adjust their sensitivity through the light or dark adaptation processes described by the photoreception model. The higher the retinal illuminance, the more the photoreceptor reduces its sensitivity to avoid saturation; conversely, the lower the retinal illuminance, the more it increases sensitivity to maintain a detectable response.

This same retinal illuminance also determines the luminance contrast required between an object and its background for the object to remain detectable: the lower the retinal illuminance, the greater the contrast needed to generate a receptor potential difference large enough to distinguish the object; conversely, the higher the retinal illuminance, the less contrast is required to achieve that same distinction. This minimum contrast required for detection thus represents a threshold that becomes more demanding as retinal illuminance decreases.

For this reason, this photometric model—which weights radiometric data against the eye's spectral sensitivity to calculate retinal illuminance and the minimum contrast required for detection—serves as the foundation for designing any lighting intended for comfortable human vision. It determines, for instance, how much light a screen must emit for its contrasts to remain detectable, or what level of illuminance must be maintained in a room where visual acuity is assessed via an optotype projector or the visual field is measured via perimetry, ensuring that the clinical results depend on the patient's vision rather than on insufficient lighting.

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