IR spectrum plotter

Upload, compare, and label infrared spectra without sending the measurement files to a server.

IR workspace

Upload up to 10 spectra

First numeric column: wavenumber · second numeric column: %T or absorbance

Files stay in your browser and are not uploaded to a server.

Infrared spectrum

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4000 → 400 cm⁻¹

Your spectrum will appear hereUpload a CSV, TXT, or XLSX file with wavenumber in the first column and measurement in the second.

How it works

The browser reproduces the plotting and peak-selection workflow from the supplied Python notebook: spectra are ordered by wavenumber, %T minima or absorbance maxima are detected, and the strongest peaks are selected by prominence and point spacing. Absorbance conversion uses A = −log₁₀(T/100). Hover inspection is available with a mouse or trackpad.

Why absorbance is logarithmic

Move the transmitted-light fraction to connect the two scales. This teaching curve is independent of your uploaded measurements.

Transmittance50%

1%100%

Absorbance · dimensionless0.301A = −log₁₀(50 / 100)

From transmitted light to absorbanceAbsorbance / dimensionless

Conversion curveSelected: 50% transmittance

Transmittance / %

A = −log₁₀(%T / 100). A tenfold decrease in transmittance adds 1 to absorbance: 100% → 0, 10% → 1, 1% → 2. The example stops at 1%; zero transmission has no finite logarithm.

View chart values

From transmitted light to absorbance
Transmittance / %	Conversion curve / Absorbance / dimensionless	Selected: 50% transmittance / Absorbance / dimensionless
1	2	0.301
2	1.699	0.301
3	1.523	0.301
4	1.398	0.301
5	1.301	0.301
6	1.222	0.301
7	1.155	0.301
8	1.097	0.301
9	1.046	0.301
10	1	0.301
11	0.959	0.301
12	0.921	0.301
13	0.886	0.301
14	0.854	0.301
15	0.824	0.301
16	0.796	0.301
17	0.77	0.301
18	0.745	0.301
19	0.721	0.301
20	0.699	0.301
21	0.678	0.301
22	0.658	0.301
23	0.638	0.301
24	0.62	0.301
25	0.602	0.301
26	0.585	0.301
27	0.569	0.301
28	0.553	0.301
29	0.538	0.301
30	0.523	0.301
31	0.509	0.301
32	0.495	0.301
33	0.481	0.301
34	0.469	0.301
35	0.456	0.301
36	0.444	0.301
37	0.432	0.301
38	0.42	0.301
39	0.409	0.301
40	0.398	0.301
41	0.387	0.301
42	0.377	0.301
43	0.367	0.301
44	0.357	0.301
45	0.347	0.301
46	0.337	0.301
47	0.328	0.301
48	0.319	0.301
49	0.31	0.301
50	0.301	0.301
51	0.292	0.301
52	0.284	0.301
53	0.276	0.301
54	0.268	0.301
55	0.26	0.301
56	0.252	0.301
57	0.244	0.301
58	0.237	0.301
59	0.229	0.301
60	0.222	0.301
61	0.215	0.301
62	0.208	0.301
63	0.201	0.301
64	0.194	0.301
65	0.187	0.301
66	0.18	0.301
67	0.174	0.301
68	0.167	0.301
69	0.161	0.301
70	0.155	0.301
71	0.149	0.301
72	0.143	0.301
73	0.137	0.301
74	0.131	0.301
75	0.125	0.301
76	0.119	0.301
77	0.114	0.301
78	0.108	0.301
79	0.102	0.301
80	0.097	0.301
81	0.092	0.301
82	0.086	0.301
83	0.081	0.301
84	0.076	0.301
85	0.071	0.301
86	0.066	0.301
87	0.06	0.301
88	0.056	0.301
89	0.051	0.301
90	0.046	0.301
91	0.041	0.301
92	0.036	0.301
93	0.032	0.301
94	0.027	0.301
95	0.022	0.301
96	0.018	0.301
97	0.013	0.301
98	0.009	0.301
99	0.004	0.301
100	-0	0.301

Reference guide

How to read the IR regions

Based on pages 1–2 of the supplied IR ranges document.

4000–2500 cm⁻¹

3 μ / X-H region

Best for O-H, N-H and C-H stretching. Shape matters: free O-H and N-H tend to be sharper, while hydrogen bonding broadens and lowers them. Very weak bands can be overtones.

2500–1800 cm⁻¹

Triple-bond region

Use for C≡N, C≡C, cumulative double bonds and some overtones. Nitriles and isocyanates are often diagnostic; symmetrical alkynes may be weak or absent.

1800–1500 cm⁻¹

6 μ / double-bond region

Usually the most useful area for C=O, C=C and C=N stretching. Carbonyl position shifts predictably with conjugation, ring strain and nearby electronegative atoms.

1500–650 cm⁻¹

Fingerprint region

Contains single-bond stretches, bends and complex coupled vibrations. Assign only stronger diagnostic bands; use the overall pattern to compare identity with a known spectrum.

Wavenumber

ν̄ = 1/λ = ν/cHigher wavenumber means higher vibrational frequency and energy.

Transmittance

T = I/I₀%T = 100TCompares transmitted intensity with the incident beam.

Absorbance

A = −log₁₀(T)Converts transmission dips into upward peaks and is proportional to concentration in the linear regime.

Beer–Lambert law

A = εbcRelates absorbance to molar absorptivity ε, path length b and concentration c.

Harmonic oscillator

ν̄ = [1/(2πc)]√(k/μ)Stronger bonds have larger force constants k and absorb at higher wavenumber.

Reduced mass

μ = m₁m₂/(m₁ + m₂)Heavier bonded atoms lower the vibrational frequency; isotopic substitution therefore shifts bands.

Vibrational levels

Eᵥ = (v + ½)hνIR absorption promotes a molecule between quantized vibrational states.

Selection rule

Δv = ±1∂μdipole/∂Q ≠ 0A fundamental vibration must change molecular dipole moment to be IR-active.

Practical rule: assign the interpretable high-wavenumber, triple-bond and double-bond regions first. Then use only selected medium or strong fingerprint peaks and confirm them through companion bands or comparison with a known spectrum. Complete assignment of every band is rarely justified.

Where the spectrum comes from

An FTIR never measures one wavenumber at a time. It slides a mirror, records how the light at the detector rises and falls, recovers every wavenumber with a Fourier transform, and divides the sample scan by a background scan. The examples here are teaching models, kept apart from your uploads.

Equal arm lengths: OPD 0 (declared reference)Forward and return paths drawn separately.

Loading the 3D view…

x = 0.00 µm · δ = 0.00 µmSample outBeam: hardware view

Hardware colorsBroadband IR sourceBeamsplitterFixed mirrorMoving mirrorMirror carriage and driveTransmission sample cellDetector and electronicsReference-laser metrology

Hardware and paths

Step 1 of 4

The interferometer modulates a mixture of infrared frequencies before the detector records their combined power.

Light from the source is split, travels out and back along two arms, recombines at the splitter and passes the sample to the detector. At the declared zero both arms are 50 mm, so the optical path difference is zero.

Mirror at the equal-arm referencemagnified rail, µm

Fixed arm, out + back: 50 + 50 mm
Moving arm, out + back: (50 + x) + (50 + x) mm
OPD δ: 2x (0 here)
Full scan: x = 1.25 mm → δ = 2.5 mm

Teaching data, not measurements.The three lines (1000, 1500, 2000 cm⁻¹; source weights 1/0.7/0.5, transmittances 0.25/0.8/0.1) are a Fourier demonstration, not a source or a molecule. The broadband spectrum is the site's synthetic ethanol-like model. CO₂ motions are schematic symmetry-derived coordinates; the 1000/1008 cm⁻¹ doublet is invented.

Hardware

Select a part in the 3D scene or from this list. Colors: amber source, purple splitter and detector, blue fixed arm, red moving arm and reference laser, teal sample, steel mechanics.

Adding final touches

From transmitted light to absorbance

Transmittance / %

Conversion curve / Absorbance / dimensionless

Selected: 50% transmittance / Absorbance / dimensionless

0.301

1.699

0.301

1.523

0.301

1.398

0.301

1.301

0.301

1.222

0.301

1.155

0.301

1.097

0.301

1.046

0.301

0.959

0.301

0.921

0.301

0.886

0.301

0.854

0.301

0.824

0.301

0.796

0.301

0.77

0.301

0.745

0.301

0.721

0.301

0.699

0.301

0.678

0.301

0.658

0.301

0.638

0.301

0.62

0.301

0.602

0.301

0.585

0.301

0.569

0.301

0.553

0.301

0.538

0.301

0.523

0.301

0.509

0.301

0.495

0.301

0.481

0.301

0.469

0.301

0.456

0.301

0.444

0.301

0.432

0.301

0.42

0.301

0.409

0.301

0.398

0.301

0.387

0.301

0.377

0.301

0.367

0.301

0.357

0.301

0.347

0.301

0.337

0.301

0.328

0.301

0.319

0.301

0.31

0.301

0.292

0.301

0.284

0.301

0.276

0.301

0.268

0.301

0.26

0.301

0.252

0.301

0.244

0.301

0.237

0.301

0.229

0.301

0.222

0.301

0.215

0.301

0.208

0.301

0.201

0.301

0.194

0.301

0.187

0.301

0.18

0.301

0.174

0.301

0.167

0.301

0.161

0.301

0.155

0.301

0.149

0.301

0.143

0.301

0.137

0.301

0.131

0.301

0.125

0.301

0.119

0.301

0.114

0.301

0.108

0.301

0.102

0.301

0.097

0.301

0.092

0.301

0.086

0.301

0.081

0.301

0.076

0.301

0.071

0.301

0.066

0.301

0.06

0.301

0.056

0.301

0.051

0.301

0.046

0.301

0.041

0.301

0.036

0.301

0.032

0.301

0.027

0.301

0.022

0.301

0.018

0.301

0.013

0.301

0.009

0.301

0.004

0.301

100

-0

0.301

Where the spectrum comes from

Equal arm lengths: OPD 0 (declared reference)Forward and return paths drawn separately.

Loading the 3D view…

x = 0.00 µm · δ = 0.00 µmSample outBeam: hardware view

Hardware colorsBroadband IR sourceBeamsplitterFixed mirrorMoving mirrorMirror carriage and driveTransmission sample cellDetector and electronicsReference-laser metrology

Hardware and paths

Step 1 of 4

The interferometer modulates a mixture of infrared frequencies before the detector records their combined power.

Mirror at the equal-arm referencemagnified rail, µm

Fixed arm, out + back: 50 + 50 mm
Moving arm, out + back: (50 + x) + (50 + x) mm
OPD δ: 2x (0 here)
Full scan: x = 1.25 mm → δ = 2.5 mm

Hardware

Select a part in the 3D scene or from this list. Colors: amber source, purple splitter and detector, blue fixed arm, red moving arm and reference laser, teal sample, steel mechanics.