The Light and Human Vision

How people see and is affected psychologically by light has been a research and discussion scheme since long time ago. Describing light as "lumens" and measuring it as "luxes on surface" has been the traditional method of description and definition of how much light is needed to carry out diverse activities.
However, this is being redefined by the results of the researches in visual effects and psychological impacts of the light. The chromatic performance rate and the color of temperature correlationated also start to be part of the description of the quality of any light. With the advance of the lighting technology, with different colors and types of lights, the simply measure of lumens cannot predict the vision quality for a human.
For example, a low pressure sodium lamp can generate a high quantity of lumens, but only can reveal two colors (yellow and grey). With this light, it can only be revealed the shape of an object, without revealing the most important thing, the detail of the object. The human vision is affected by many factors,from lighting intensity, distribution or color, to contrast, reflection, dazzle, aerial quality, position and movement of the object etc. Our eyes use different parts to see the object at high light conditions or low light conditions.
Human eyes have cones and rods which have been designed to work in opposite conditions.
The cones provide the color vision and detail in high light conditions and the rods make it in low light conditions. In high light conditions, our pupils are contracted, observing more details of the object, while the field depth and the brightness also grow. In low light conditions, pupils are dilated to allow more entry of light. Optical and lighting levels measuring instruments recommended have been calibrated traditionally for daytime seeing, and indoor lighting in general. However, so many researches prove that the theory of vision under high light conditions and low light conditions is applied more than considered. In lots of recent references, researchers encourage the lighting designers to specify the reason between high light conditions and low light conditions (Reason F/E) when lamps were chosen to have more design and visual efficiency to the customers.
Sam Berman, member of the Lawrence Berkeley laboratory lighting research group, is one of the researchers who insist in applying the reason F/E in the lamp selection. Applying the reason F/E, he developed a conversion factor which obtained the efficient lumen perceived by human eyes in different conditions of pupil size and visual effects by the exit of the different lamps. Some lamps, like low pressure sodium lamps, lose the main lumen flux exit applying this theory, while in the hiLED lamp, the effect is ideal.
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