Learn About Light

 

A short education on color, infrared and ultraviolet light.

 

Color can be defined by particular wavelength(s) of light.  A pure color will have one particular wavelength such as 450nm will appear as a vibrant blue.  The nm stands for nanometer and is a measure of length.  One nanometer is a billionth of a meter.  The measurement is for one full light wave length. 

 

 

Humans can see from about 400nm (violet) to 750nm (red).  Below 400nm is ultraviolet light.  Above 750nm is infrared.

 

The human eye is not equally sensitive to all colors.  We see color best during the day when we are using the cone sensors in our retinas.  Below is a graph showing how the cone eye sensors are more sensitive at some wavelengths than others.

 

 

The human eye also changes in color sensitivity depending on the ambient light.  During the day, the eye uses cone sensors in retina which are color sensitive and high resolution.  At night, the eye switches to rod sensors which are not very color sensitive.  At very low illumination levels the eye cannot see color at all.

 

The eye is most sensitive in the green - yellow portion at around 550nm at night when it is using its rod sensors.  

 

 

 

 

So where does the human eye see relative to other wavelengths?  Below is a graph showing the human range of the electromagnetic spectrum and where it fits relative to other bands.  

 

 

 

 

Ultraviolet Classifications

 

bullet UV-A: 320nm to 400nm
bullet UV-B: 280nm to 320nm
bullet UV-C: 100nm to 280nm

 

 

 

Infrared Classifications

bullet Near-IR: 750nm to 3000nm (0.75 to 3.0 microns)
bullet Mid-IR: 3,000nm to 30,000nm (3 to 30 microns)

 

  1. Mid-Wave IR (MWIR) 3,000nm to 8,000nm (3 to 8 microns)

  2. Long-Wave IR (LWIR) 8,000nm to 12,000nm (8 to 12 microns)

bullet Far-IR: 30,000nm to 350,000nm (30 to 350 micons)

 

 

 

Your typical infrared enabled digital camera (e.g. Sony Camera with NiteShot), can see between 400nm and 1100nm.  The non-human visible range between 750nm and 1100nm is called the Near IR (NIR) range.  The NIR does not see heat.  Heat is seen in the 8,000nm to 13,000nm range called the Long-Wave (LWIR) range.

Some example uses of UV and IR lights and cameras
    - Using a UV flash with a camera that can see infrared and fitted with an IR longpass filter.  Some faded inks and hard to see writing will fluoresce in the infrared when excited with a UV light.
    - Using UV lights to see old signatures.
    - Using IR lights with an IR sensitive camera to see graphite signatures on wood
    - Using visible lights with an IR  sensitive camera to see the infrared fluorescence of Egyptian Blue pigment.  Tiny amounts of pigment that can't normally be seen can be visible.
    - Using UV lights to determine the float side of glass
    - Using UV and IR lights to see repair work on old rugs
    - Using IR lights to see through layers on paintings