Absorption/Reflection

[Andy Finney <andy@xxxxxxxxxxxxxxxx>]

Dirk Wright wrote:

I was wondering about infrared reflectance and absorption of various
materials. I shot some EIR recently, and materials like brick, gravel,
and mulch recorded as not reflecting (ie, not red). If a material does
not reflect IR, can it be assumed that it absorbs light at that
frequency? Does this further mean that these materials will soak up
heat, or rather get hotter, than materials that reflect IR, given
identical solar illumination? This can have implications for housing
and landscaping design beyond the normal artistic effects.

And my take on this is ....

Surely it's relatively simple. If a substance does not reflect 
radiation (whether visible or not) then the energy of the radiation 
has to go somewhere. Usually it heats up the object but in some 
(rare) cases it will generate an electrical potential difference or 
some other equally curious effect.

Clark says that objects with temperatures of between 250 Celsius and 
500 Celsius can be photographed by their own illumination using 
infrared film. Above about 600 Celcius the objects are hot enough to 
glow visibly. A chapter in Clark is devoted to the characteristics of 
materials in infrared light including some charts. From that I 
appropriate the following bits and pieces:

For example: Water has a pronounced dip in infrared absorption at 
around 800nM, but it is still three times as absorbent as at 700nM. 
This dip gets more pronounced as the water heats up mainly because 
the absorption around the dip increases.

Substances such as copper and gold show around 90% reflectivity at 
800nM whereas at 400 they are only 30% reflective. (This makes sense 
if you consider what colour gold and copper appear to be in visible 
light.) Carbon on the other hand (as graphite) has only a very slight 
rise - almost a straight line - from 400 to 1400nM. Aluminium is 
almost constant at about 85% from below 400nM to above 1400nM. 
Chromium has a shelf of sensitivity which rises from around 40% at 
400nM to 55% at 600nM. It then stays virtually constant up to 1400 
and perhaps beyond.

For building materials the reflection generally stays constant (+/- a 
few percentage points) between 500nM and 840nM but often climbs at 
1780nM. Exceptions to this include clay tiles, which steadily climb 
in reflectivity as the wavelength gets longer.

Snow has an undulating reflective curve: 35% at 300-400, 40% at 
400-800, 15% at 800-2600 ... only climbing again above 7000nM.

Regards

Andy
www.invisiblelight.co.uk
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