BLOG: Colour - Differences & Similarities
A rather obvious statement, for of course, red is completely different to green. Likewise, blue differs to pink. The use of dissimilar colours can make objects and scenes appear completely different from an aesthetic perspective. But when considering laser exposure, what difference does colour make from a safety perspective?
Lasers that emit visible light have the potential to cause thermal injuries to a part of the eye called the retina. Excess light exposure causes a rapid heat rise in the eye, leading to a small irreversible burn in a fraction of a second. It doesn’t matter if the colour of the light is red, green, or blue in appearance, the same quantity of any of these colours at excess levels will cause the same injury. That is to say, 1 watt of red light will cause the same injury as 1 watt of green light, when delivered in the same concentration.
But not the same…
A complication arises because the eye perceives different colours differently, even if the concentration of two the different colours are identical. For example, 1 watt of red light and 1 watt of green light could be present in a two separate beams in exactly the same concentration. But when viewing these, the red will look much dimmer in appearance to the green beam. This not because the red is weaker, or the green stronger, but instead because the eye is far less sensitive to red than green light, so it appears much dimmer.
This is important for safety, because it means that the human eye can’t be used to make comparisons about power levels, just by looking at different colour laser beams. A power meter is the only way to establish what levels are present in the beam.
The implication from this is that to have dimmer perceived colours such as red and blue look a similar brightness to green light, much higher power levels must be used on these colours to achieve a match, leading to the exposure levels from the red and blue light potentially being more hazardous, even though the lighting levels look the same.
Outdoors, the visual effect is also more critical as green light appears brighter than blue and red, so has a greater potential to cause dazzle at distances far beyond the distance at which thermal injuries may occur (the NOHD).
In some parts of the world, aviation control agencies take the colour into consideration when determining safe distances for lasers to be used near aircraft.
Blue light exposure
Thermal injuries are the more dominant injury type for exposure from too much visible light. This is because they normally happen very quickly; fractions of a second for Class 3B and Class 4 laser exposure. However for longer term blue light exposure at lower levels, a different type of injury can occur which is photochemical, (similar to sun burn). This means that exposure to dim looking blue light, that may be below the thermal injury threshold, and maybe not having the dazzle risk of green and yellow light, can present a different type of risk, that may need to be considered for certain exposure scenarios.
© 2015 LVR Optical