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Technical Information:
Working with Optical Density |
Optical
Density – or OD, as it is commonly called – is a convenient
tool to describe the transmission of light through a highly blocking
optical filter (when the transmission is extremely small). OD is
simply defined as the negative of the logarithm (base 10) of the
transmission, where the transmission varies between 0 and 1 (OD
= – log10(T)). Therefore, the transmission is simply 10 raised
to the power of minus the OD (T = 10 – OD). The graph below
left demonstrates the power of OD: a variation in transmission of
six orders of magnitude (1,000,000 times) is described very simply
by OD values ranging between 0 and 6. The table of examples below
middle, and the list of “rules” below right, provide
some handy tips for quickly converting between OD and transmission.
Multiplying and dividing the transmission by two and ten is equivalent
to subtracting and adding 0.3 and 1 in OD, respectively.
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Transmission OD
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1
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0
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0.5
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0.3
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0.2
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0.7
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0.1
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1
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0.05
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1.3
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0.02
0.01
0.005
0.002
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1.7
2
2.3
2.7
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0.001
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3
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The "1" Rule
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T
- 1 –> OD - 0
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The "x
2" Rule
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The" ÷ 2" Rule
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T ÷ 2 –> OD
+ 0.3
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The "x
10" Rule
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T
x 10 –> OD -
1
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T
: 10 > OD | 1
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