Specific detectivity D* expresses how sensitively a detector converts incident power into a measurable signal, normalized to detector area and noise bandwidth.
- D* = √A / NEPdensity where NEPdensity is in W/√Hz.
- If you have an RMS noise-equivalent power over a bandwidth: NEPdensity = NEPrms / √Δf.
- With responsivity R (A/W) and noise current density in (A/√Hz): NEPdensity = in/R.
- With voltage noise density vn (V/√Hz) and transimpedance gain Zt (V/A): in = vn/Zt, then NEPdensity = (vn/Zt)/R.
- Select the method that matches the data you measured.
- Enter the detector active area and choose its unit.
- For RMS noise values, enter the noise bandwidth Δf in Hz.
- Provide NEP, or responsivity with the appropriate noise input.
- Click Calculate to view D* above the form.
- Use Download CSV or Download PDF to export your result.
| Case | Area (cm²) | Method | Key input | NEP density (W/√Hz) | D* (Jones) |
|---|---|---|---|---|---|
| 1 | 0.50 | From NEP | NEP = 2.0e-12 W/√Hz | 2.0e-12 | 3.54e11 |
| 2 | 0.20 | Resp + current noise | R = 0.6 A/W, in = 1.0e-12 A/√Hz | 1.67e-12 | 2.68e11 |
| 3 | 1.00 | Resp + voltage noise | R = 0.7 A/W, vn = 4.0e-9 V/√Hz, Zt=1.0e4 V/A | 5.71e-13 | 1.75e12 |
1) What D* represents
D* (D-star) summarizes how well a detector can resolve weak optical signals in noise. It scales with √active area and inversely with noise-equivalent power per √Hz. Higher D* indicates better sensitivity for the same measurement bandwidth. It is commonly referenced at SNR = 1 in a 1 Hz bandwidth.
2) Jones versus SI reporting
Datasheets often report D* in Jones, defined as cm·√Hz/W. The same quantity in SI is m·√Hz/W. Because 1 m = 100 cm, mixing units shifts the number by 100, so keep units consistent when comparing devices.
3) Connection to NEP density
The core relation is D* = √A / NEP using NEP spectral density in W/√Hz. If you only have an RMS NEP across a bandwidth, the tool converts it using NEP_density = NEP_rms / √Δf. This helps normalize spectrum or lock-in readouts.
4) Responsivity-driven workflows
Many setups measure responsivity R (A/W) and electrical noise instead of NEP. With current noise density in, use NEP = in/R. Example: R = 0.6 A/W and in = 1.0×10-12 A/√Hz gives NEP ≈ 1.67×10-12 W/√Hz.
5) Voltage noise and transimpedance stages
For transimpedance amplifiers, you may know input-referred voltage noise density vn and gain Zt (V/A). Convert to current noise with in = vn/Zt, then compute NEP and D*. This highlights whether electronics dominate.
6) Bandwidth, modulation, and comparison fairness
Noise densities are normalized per √Hz, while RMS noise grows with √Δf. Two systems with equal density can show different RMS noise if bandwidth differs. Use Δf that matches your filter or lock-in equivalent noise bandwidth.
7) Typical magnitudes and quick sanity checks
Across wavelengths and temperatures, many detectors fall around 109 to 1013 Jones, with specialized devices higher. If your output is extreme, recheck area units and whether your noise input was RMS or density. Thermal noise, shot noise from dark current, and readout noise can dominate in different regimes.
8) Using D* to guide design choices
D* supports engineering tradeoffs: increasing area raises √A but can increase capacitance and amplifier noise. Improving responsivity raises D* linearly, while reducing electrical noise improves it inversely. When comparing candidates, keep wavelength and temperature fixed, and record the bandwidth. Use the CSV/PDF exports to track iterations as you change detector size, bandwidth, or readout gain.
1) What is the “best” D* value?
Higher is generally better, but “best” depends on wavelength, temperature, bias conditions, and bandwidth. Compare devices only under similar test conditions and verify whether the value is quoted in Jones or SI form.
2) Why does this calculator ask for detector area?
D* normalizes sensitivity by area: doubling area increases √A, which raises D* if NEP density stays constant. However, larger area can also increase capacitance and noise, so D* is not always improved in practice.
3) When should I enter bandwidth Δf?
Enter Δf when your NEP, current noise, or voltage noise is provided as an RMS total across a bandwidth. If your noise value is already a spectral density (per √Hz), Δf is not required.
4) How do I choose between NEP and responsivity methods?
If you have a measured or specified NEP density, use the NEP method directly. If you instead have responsivity plus electrical noise (current or voltage), choose the matching responsivity method so the calculator can derive NEP consistently.
5) My D* seems too high. What should I check?
Confirm that area units are correct (mm² vs cm² vs m²), and that noise numbers are not mistakenly treated as densities. Also verify transimpedance gain units (V/A) and that responsivity is in A/W, not V/W.
6) Does D* depend on frequency?
Yes. Noise sources and responsivity can vary with frequency, and bandwidth choices change RMS noise. Datasheets often specify D* at a particular modulation frequency and measurement bandwidth, so keep those conditions aligned when comparing results.
7) What does the NEP density output mean?
NEP density is the optical power spectral density that produces a signal-to-noise ratio of one in a 1 Hz bandwidth. It is expressed as W/√Hz and is the denominator in the D* calculation once area is normalized.