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Vision

A simple definition of light is visually perceived radiant energy. This visible light is a small part of the electromagnetic spectrum and ranges in wavelengths from 380 nm to 780 nm. Light is what energizes our visual system. Light reflected from objects  into our eyes enables us to see.

Lightwaves enter the human eye and send signals along the optic nerve.

Light enters the eye through the pupil, is focused on the retina, and is transmitted to our brain via the optic nerve. A significant part of our brain is dedicated to the processing of visual information.

Of all creatures on this earth, the human visual system is the most impressive. Birds may have sharper vision, felines may be more sensitive to motion, and horses may have almost a 360-degree field of view, but nothing can beat the combination of features humans possess.

The human eye allows good depth perception, excellent color perception, reasonable motion detection, and great visual acuity. We're going to concentrate on just two aspects for now, mainly light and color. 

Color Perception is Subjective

One of the misconceptions about the perception of light is the fact that it is highly subjective. It is impossible to prove that any two people see a particular object in exactly the same way. This makes it a real challenge to scientifically measure certain attributes of lighting such as color and quantity.

We all tend to think of an object as having fixed colors. In reality, an object's appearance results from the way it reflects the particular light falling on it. Under pure white light, a green apple appears green because it reflects the green portion of the spectrum and absorbs the rest.

Reflection of Light

You might say the evaluation of color is somewhat subjective. What is the color of that green apple? One may say bright green; another light green;  and yet another simply green.  Who is correct? 

The same thing can be said of the quantity of light. While it is fairly easy to determine when there is relatively more or less light on a given object or in a space, actual measurements must take into account the way the eye sees and how it responds to each color of the visible spectrum.  Now let's look at the link between light and color. In fact, you could say that light is color. 

The full Electromagnetic Spectrum

Above Though invisible, radio waves are also a part of the electromagnetic spectrum.

Concepts

A Few Lighting Metrics

Since we've learned that lighting is subjective - perceived differently from person to person - terminology and standards of measurement are needed to communicate information about lighting.

LUMEN

The lumen is the unit of measure describing the quantity of light emitted from a source of light.

WATT

A watt is a unit of power denoting the rate at which electricity is used.

LUMENS PER WATT

Put the two together, and you get LUMENS PER WATT, also called "efficacy" (often wrongly referred to as efficiency). It is similar to "miles per gallon" for an automobile and is a measure of how effective the light source is in turning the watts we put in into the light we want out.

AVERAGE RATED LIFE

This term (we'll shorten it from now on and just say "life") is a statistical way of determining how long lamps will last before burning out. It's really not an average; it's the median, but the term average is more familiar.

Spotlight Night Vision Vs. Daytime Vision

Vision Diagram

Have you ever heard the term "night vision or daytime vision?" We actually use a different part of the eye to see help us see color during the day, vs. seeing shapes and objects without color at night. Here's how it works. 

The retina contains two classes of light receptor cells: rods and cones, which are named for their shape. The typical human eye has about 120 million rods and about 8 million cones.  These receptors are distributed somewhat differently throughout the eye and perform different functions.

Daytime Vision

The cones are responsible for high-level daytime photopic vision, meaning they are active in relatively high light levels and are responsible for our color vision. There are three types of cones, each sensitive to different wavelengths (colors) of light.  

Color Blindness

If one of the three cone types is deficient or absent in a person's eye, that person is referred to as being "color blind" (i.e., lacks the ability to distinguish certain colors), or is referred to as having color deficient vision, depending on the severity of the altered color vision. Approximately 8% of the human male population and 0.2% of the female population has some form of color blindness.

Nighttime Vision

The rods, which operate under very low light levels, are responsible for low-level nighttime, or scotopic, vision and respond somewhat differently to light than do the cones.  Because there is only one type of rod, these receptors can provide only information on the amount of light leaving a surface-not on the color of this light.  Therefore, scotopic vision, which occurs when only rods are active, does not allow us to see color.  This is the reason why it is impossible to determine the color of an article of clothing in a very dark room.  

At Dawn and Dusk

There is a range of luminance levels where the response of our eyes is neither photopic nor scotopic.  Under these conditions, our cones are beginning to shut down, and our visual response to colors lies somewhere between the two response curves.  This is referred to as the mesopic range, and represents how we see under many typical outdoor nighttime conditions, such as driving on a highway or walking in a very dimly lighted parking lot.  Our ability to see visual details is somewhat reduced compared to what it is under photopic vision, but it is still better than under pure scotopic vision.