The risk of losing your eyesight from accidental exposure to laser radiation is due to the special optical properties of the human eye. When we consider the different depths of penetration in relation to the wavelengths we see that the eye is transparent only in the wavelength range between 380 and 1400 nm.

UV-light below 350 nm advances to the lens or is absorbed at the surface of the eye.

A consequence of exposure to high power light at these wavelengths is an injury to the cornea by ablation or a cataract.

Light in the visible wavelength region (380-780 nm) advances to the retina. The eye is sensitive to radiation and humans have developed natural protective mechanisms. When the light appears too bright, which means the power density exceeds a damage threshold of the eye, we automatically turn away and close our eyes (i.e. aversion response or blink reflex). This automatic reaction is effective for radiation up to 1 mW power. With higher power levels, too much energy reaches the eye before the blink reflex can respond, which can result in irreversible retinal damage.

The near infrared wavelengths (780 - 1400 nm) are a type of radiation that is especially dangerous to the human eye because we have no natural protection against it. The radiation advances to the retina, but the exposure is only noticed after the damage is done. Infrared radiation (1400 - 11000 nm) is absorbed at the surface of the eye. It leads to overheating of tissue and burning, or ablation of the cornea.

Daylight Transmission (VLT) and Color Vision

When we wear laser safety glasses some wavelengths of the spectrum that would normally reach our eyes are filtered out. If we block light from the visible region, this inevitably changes our perception of our environment. First, by attenuation of the transmission the environment gets darker (similar to the effect of sun glasses). Second, blocking some wavelengths changes our perception of color.

VLT

The attenuation of light by a filter in the visible spectrum is defined as the visible light transmission (VLT) or luminous transmittance. VLT is determined in relation to a standard illuminant and evaluated according to the spectral sensitivity of the the eye to daylight-adapted (photopic) or night-adapted (scotopic) and is described in the ANSI standard Z136.7 for Testing and Labeling of Laser Protective Equipment.

Should measured VLT-value be less than 20%, the user should ensure that their working environment receives additional illumination. With a low VLT and bad illumination one can expect our eyes to adapt to so-called night vision. In doing so the color vision is restricted and the spectral sensitivity of the eyes moves towards the shorter wavelengths. For these kinds of filters it is also useful to provide the VLT-value for night vision

Color Vision

Since our eyes can adapt to different light situations and the total amount of light can be balanced by additional illumination, another important aspect for the selection of a laser safety filter is color vision. If color vision is impaired or restricted, some colors may not be recognized. This effect may also apply to warning lights or displays, or the ability to distinguish between instruments or vessels marked by color such as those found in medical surroundings.