Optical contrast in decibels ($L=10{\log }_{10}C$)

Optical contrast in decibels expresses a specified ratio logarithmically. For peak-to-local radiant power contrast $C=P_{\max }/P(t)>0$,The inverse convention $r=P(t)/P_{\max }$ gives $10{\log }_{10}r=-L$. State the convention and the time of the weak feature. A $60\mathrm{dB}$ peak-to-feature contrast means one million to one in power, not in field amplitude.

An original readout example uses a linear photodetector whose baseline-corrected output is proportional to optical power. A main reading of $0.80\mathrm{V}$ and weak-feature reading of $80\mu \mathrm{V}$ imply $C=10^4$ and $L=40\mathrm{dB}$. Applying $20{\log }_{10}$ to those readings would incorrectly report $80\mathrm{dB}$: the electrical signal represents power, not optical wave amplitude.

If instead a measurement is proportional to an optical field amplitude $A$, with the same impedance and spatial normalization for both fields, power is proportional to $|A{|}^2$. ThenA field-amplitude ratio of $100$ and a power ratio of $10^4$ therefore express the same $40\mathrm{dB}$ contrast. The correct multiplier follows the measurement model, not whether the readout unit happens to be volts.

Contrast does not specify pedestal energy fraction, optical prepulse duration or delay coverage. The Sequoia primary sheet specifies dynamic range, scan range and optical temporal resolution separately.^[ref] A signal beneath a known measurement floor supports a contrast bound within that window; subtracting an uncertain background does not establish unlimited contrast.