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Optical Density, Absorbance and Percent Transmission

Relate matching incident/transmitted light readings, decadic optical density, absorbance and percent transmission with guarded inverse entry.

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Calculator inputs

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How to use this calculator

  1. Enter the known values in the units shown. Results update as you type.
  2. Where results are editable, change one to solve backwards. Lock a value to hold it fixed.
  3. Use the worked example to check the method. Reset restores the starting fields.

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Formula and method

Incident reading 100 and transmitted reading 10 give 10% transmission and decadic optical density 1. With absorption-only measurements, absorbance is also 1.

T = I/I₀; transmission (%) = 100T; decadic OD = −log₁₀(T). For an absorption-only measurement, A = OD.

Worked example

Enter these known values and leave the other values blank.

Incident light reading I₀
100
Transmitted light reading I
10
Decadic optical density
1
Absorbance for absorption-only readings
1
Light transmission
10 %

Assumptions and limitations

  • Use matching positive incident and transmitted readings in the same units and spectral measurement setup; the transmitted reading cannot exceed the incident reading in this passive attenuation model. Their absolute unit cancels in the ratio.
  • Transmission uses percent numbers internally and in its control: enter 10 for 10%. Finite optical density requires transmission greater than 0% and at most 100%. Perfect opacity (0% transmission) corresponds to an infinite logarithmic result and is outside this finite model.
  • The logarithm has base 10. These dimensionless values are not natural-log optical depth or a physical density of mass per volume.
  • Optical density can describe total attenuation. Interpreting the same ratio as absorbance requires readings that exclude reflection and scattering effects; this tool equates the two only within that absorption-only interpretation.
  • A transmission or logarithmic value alone determines the other ratios, but cannot determine an absolute incident or transmitted reading. Add one light reading to recover the other.
  • This model does not infer chemical concentration. A Beer–Lambert concentration calculation additionally requires an appropriate path length, absorption coefficient and measurement conditions.

Common questions

Why does 10% transmission give optical density 1?

Ten percent is the fraction 0.1; the negative base-10 logarithm of 0.1 is 1.

Can I start with percent transmission?

Yes. Enter a positive transmission up to 100% to calculate the logarithmic results. Add one light reading to recover the other; absorbance and optical density are computed output fields.

References

Require positive transmission no greater than 100 percent and nonnegative finite decadic optical density/absorbance. Correct the captured optical-density sign, retaining the source percent-based computation scale and its factor 100. Express the equivalent OD/absorbance/transmission identities through the editable transmission control while preserving both absolute light controls, the two computed logarithmic outputs, and guarding the logarithm and dividing inverses.

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