Advanced Interstellar Extinction Calculator

Calculator Inputs

Large screens use three columns, smaller screens use two, and mobile uses one.

White theme layout

Band Preset

Choose a band or override with a custom wavelength.

Custom Wavelength (nm)

Valid CCM89 range: 100 to 3333 nm.

E(B-V)

Color excess caused by dust reddening.

R_V

Typical diffuse interstellar medium value is 3.1.

Observed Magnitude

Optional apparent magnitude at the chosen band.

Observed Flux

Optional flux value for dereddened flux recovery.

Distance (pc)

Used for distance modulus and absolute magnitude.

Observed B-V

Optional color index for reddening cross-check.

Intrinsic B-V

Optional unreddened color index estimate.

Example Data Table

Example values below use E(B-V) = 0.20 and R_V = 3.10, giving A(V) = 0.62 mag.

Band	Wavelength (nm)	Approx. A(λ)/A(V)	Approx. A(λ) (mag)	Transmission
U	365	1.57	0.97	0.41
B	445	1.32	0.82	0.47
V	551	1.00	0.62	0.57
R	658	0.75	0.47	0.65
I	806	0.48	0.30	0.76

Formula Used

1) Visual extinction
A(V) = R_V × E(B−V)

2) Extinction at wavelength λ
A(λ) / A(V) = a(x) + b(x) / R_V, where x = 1 / λ(μm)

3) Magnitude correction
m₀ = m_obs − A(λ)

4) Flux transmission and correction
Transmission = 10^−0.4A(λ)
F₀ = F_obs × 10^0.4A(λ)

This page uses the Cardelli, Clayton, and Mathis style extinction curve across near-infrared, optical, and ultraviolet wavelength regions.

How to Use This Calculator

Select a preset photometric band or enter a custom wavelength in nanometers.
Enter E(B−V) and R_V to define the dust reddening environment.
Optionally enter observed magnitude, observed flux, distance, and color-index values.
Press Calculate Extinction to generate results above the form.
Review A(V), A(λ), transmission, corrected magnitude, and dereddened flux.
Use the Plotly graph to inspect how extinction changes with wavelength.
Download the summary as CSV for spreadsheets or PDF for reports.

Frequently Asked Questions

1) What does interstellar extinction measure?

It measures how dust and gas reduce and redden light between a source and the observer. Shorter wavelengths usually suffer stronger attenuation than longer wavelengths.

2) Why is E(B-V) important?

E(B-V) is the color excess between blue and visual bands. It quantifies reddening and anchors the extinction calculation for other wavelengths.

3) What does R_V represent?

R_V is the ratio A(V)/E(B-V). It controls curve shape and reflects dust grain properties. Diffuse Milky Way regions often use about 3.1.

4) What is the difference between A(V) and A(λ)?

A(V) is extinction in the visual band. A(λ) is extinction at a specific wavelength, scaled by the extinction law and your chosen R_V.

5) Why does the calculator ask for observed flux and magnitude?

These fields are optional. Magnitude allows dereddened magnitude estimates, while flux allows recovery of intrinsic flux using the extinction correction factor.

6) Can I use a custom wavelength instead of a photometric band?

Yes. Choose the custom option and enter the wavelength in nanometers. The calculator then evaluates the extinction law at that exact wavelength.

7) Why is there a valid wavelength range?

The implemented law is defined over a limited inverse-wavelength interval. This file accepts approximately 100 to 3333 nm for consistent calculations.

8) What does the transmission fraction tell me?

It shows the fraction of light that remains after extinction at the chosen wavelength. Lower values indicate stronger attenuation by interstellar dust.