Compute optical density (OD) from transmittance and vice versa, solve the Beer–Lambert law (A = ε·c·l) in four directions, and sum the OD of stacked ND filters. All computation 100% in your browser.
What is optical density (OD)?
Optical density (OD) is a measure of how strongly a material absorbs or blocks light, defined as OD = −log₁₀(T), where T is transmittance (the fraction of light transmitted, 0 to 1). A perfectly clear material has T = 1 so OD = 0. A material transmitting only 10% of light (T = 0.1) has OD = 1.0, and one transmitting 1% of light (T = 0.01) has OD = 2.0.
What is the difference between optical density and absorbance?
The two are fundamentally the same quantity: absorbance A is also defined as −log₁₀(T). The difference is only usage context: the term “absorbance” is used in spectrophotometry/analytical chemistry with the Beer–Lambert law (A = ε·c·l), while “optical density” is more common in optics, photography (ND filters), and microbiology (OD₆₀₀). This calculator treats both identically in definition mode.
Why does the formula use −log₁₀ instead of transmittance directly?
The logarithmic scale turns stacked-filter computation into simple addition, not multiplication. Example: two filters with OD 0.3 and 0.6 combine to total OD 0.9 — without logarithms you would have to multiply transmittances 0.5012 × 0.2512 = 0.1259. Total OD 0.9 directly gives transmittance 10^(−0.9) = 12.59%, the same result far easier to compute.
What do ND2, ND4, ND8 filters mean in optical density?
The ND number is the inverse of transmittance: ND2 means T = 1/2, so OD = log₁₀(2) = 0.3010 (transmits 50% of light). ND4 → OD = log₁₀(4) = 0.6021 (25%), ND8 → OD = log₁₀(8) = 0.9031 (12.5%), ND16 → OD = 1.2041 (6.25%), ND64 → OD = 1.8062 (1.5625%), ND100 → OD = 2.0 (1%), and ND1000 → OD = 3.0 (0.1%). This calculator provides these presets in filter stack mode.
How does the Beer–Lambert law work in this calculator?
The Beer–Lambert law states A = ε·c·l: absorbance (A) equals the molar extinction coefficient (ε, in L/(mol·cm)) times concentration (c, in mol/L) times light path length (l, in cm). Mode 2 can solve all four directions: from A = 1, ε = 1000, and l = 1 cm, concentration c = 1/(1000×1) = 0.001 mol/L (equal to 1 mmol/L). The calculator converts mmol/L and mm units automatically.
Real example: how do you compute concentration from measured absorbance?
Classic example: NADH measured at 340 nm with ε = 6220 L/(mol·cm) and a 1 cm cuvette. If measured absorbance A = 0.622, then c = A/(ε·l) = 0.622/(6220×1) = 0.0001 mol/L = 0.1 mmol/L. Enter these values in Beer–Lambert mode with solve direction “c” to get the same result.
How do you compute total OD for several stacked filters?
Just sum each filter's OD: OD_total = OD₁ + OD₂ + … . Example: OD 0.3 + 0.6 + 0.9 filters give total OD 1.8, so total transmittance = 10^(−1.8) = 1.5849%. Mode 3 of this calculator accepts 1–5 filters at once and shows total OD, total transmittance, and each filter's percent transmittance contribution.
How do you read the OD vs percent transmittance table?
Some easy reference points: OD 0 → 100%, OD 0.1 → 79.43%, OD 0.3 → 50.12%, OD 0.5 → 31.62%, OD 0.6 → 25.12%, OD 1 → 10%, OD 1.3 → 5.01%, OD 2 → 1%, OD 3 → 0.1%, and OD 4 → 0.01%. Each OD increase of 1 means transmittance drops 10-fold. The full table is available on this page.
Bisakah optical density bernilai negatif?
Yes. Negative OD means transmittance greater than 1 — occurring in light-amplifying media, e.g. the gain medium in a laser. In photography and lab practice, transmittance is always between 0 and 1 so OD ≥ 0. This calculator accepts OD from −3 to 6 in definition mode so gain cases are covered too.
What is OD₆₀₀ used in microbiology?
OD₆₀₀ is the optical density of a bacterial culture measured at 600 nm wavelength, used to estimate cell density. The rough rule: OD₆₀₀ = 1.0 equals about 8×10⁸ E. coli cells per mL. Note that OD₆₀₀ mostly measures light scattering by cells, not pure absorption, so exact figures need a calibration curve for each organism and instrument.
This calculator runs fully in your browser — no data is sent to any server.