Free Black Mirror (Claude Glass)
The Claude glass was a landscape painter's instrument: a small, dark, convex mirror that stripped a scene down to its big value masses. Use it on a photo or live through your camera, as a painterly curve or as a physical model of black glass, and judge the design before the detail seduces you. Exports PNG. Free, with no sign-up, and your picture never leaves your device.
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Live through the camera.Turn your back to the scene, as painters did, and see it in the mirror as you move.
Physical model.The Fresnel reflectance of black glass: about 4% of the light, a little more toward a convex rim.
Painterly model.A darkening curve that sinks halftones into their masses, with colour pulled slightly toward grey.
Convex curvature.Bulges the middle and squeezes the edges; off gives a flat black mirror.
- Free
- No sign-up
- Runs in your browser: your image never leaves your device
What is a Claude glass?
A Claude glass is a small, dark, slightly convex mirror that eighteenth-century landscape painters and tourists used to view a scene as a dim, reduced reflection. It is named after the painter Claude Lorrain. The reflection hides detail and leaves the big value masses, and this Claude glass app does the same to a photo or a live view.
How to use the Black Mirror (Claude Glass)
The tool takes 5 steps, all on your device.
Choose what to look at: a photo (drop one in or pick a sample) or the camera, live.
Pick the model: Painterly darkens with a curve; Physical shows what black glass actually reflects.
Set Mirror strength, or for the physical model the eye's adaptation, and add Convex curvature to bulge the middle like a curved glass.
Compare with the original in Split or Flicker view, or freeze the live view.
Export the mirror view as a PNG at full resolution, or download the frozen camera frame.
What the Black Mirror (Claude Glass) does
The Black Mirror shows the big masses first, models what black glass reflects and the eye's own part, and runs live on the device.
The big masses, first
The mirror darkens a scene until detail sinks and only the large masses remain, so a composition is judged on its design. Among the value and tone tools for artists, it does the work of squinting at a strength you can set and repeat.
What black glass reflects
A blackened glass only returns the light its front surface reflects. For ordinary glass, with a refractive index of 1.52, that is 4.3% straight on by the Fresnel equations, about 4.5 stops less than the scene, and slightly more where a convex surface tilts toward the rim. Colour is unchanged, because glass reflects every colour almost equally.
The eye's part
Looking into the glass, the eye adapts to the dim image. Eye adaptation sets how much of the lost brightness it recovers: at 0% you see the true reflection, at 100% the full brightness returns. Somewhere between is what painters saw: dark enough that shadows merge, bright enough that the lit masses read.
Live, on the device
The camera view runs as a WebGL shader with the same maths as the photo path, including the convex bulge. Without WebGL it runs on the processor at a smaller size. The video never leaves your device.
Preview and export agree
The same transform renders the 1,600-pixel preview and the full-resolution PNG, so the export matches the view you judged.
Private by construction
Photos and camera frames are processed in your browser and saved on your device. Nothing is uploaded.
For painters who judge the masses before the detail
Landscape painters, working painters and art teachers judge the masses in it before the detail.
Landscape painters
Hold the phone up with your back to the view and choose the composition whose masses hold in the glass.
Working painters
Photograph a canvas mid-session and see whether its masses still match the reference.
Art teachers
Show a class, live, how a scene simplifies in a black mirror and why.
What a Claude glass actually did
A Claude glass was used with your back to the view: you held it up and studied the reflection. Museum examples, such as one in the Victoria and Albert Museum dated 1775–1780, are slightly convex, blackened mirrors. The dark surface lowered the brightness so detail faded, and the curve shrank a wide view into one small picture. What remained was the arrangement of dark and light masses that decides whether a painting holds together from across the room.
Painterly or physical
The painterly model is designed to feel like the instrument: a curve that darkens and compresses values, shadows most, with colour pulled partly toward grey. Treat its strength as how much squint, not as a measurement.
The physical model starts from the glass. Black glass reflects only what its front surface returns, a little over 4% of the light, the same for every colour. A convex glass tilts toward its rim, and reflectance rises slightly there. The rest of the effect happens in the eye: an image several stops darker than its surroundings loses its shadows first. Lower the eye adaptation to see how dark the glass really is.
Why no tinted glasses
Accounts describe Claude glasses as dark, and some as tinted, but we found no source that records the colours of particular glasses well enough to reproduce them. Rather than invent tints, the tool offers neutral black glass, which is what the physics gives, and the painterly curve.
Using it on references and work in progress
Run each candidate reference through the glass before you commit, and ask whether the scene still reads as three or four masses with a focal area that pulls the eye. When the mirror view turns to porridge, rendering skill won't rescue the composition. In the mirror view of a photographed painting, ask whether the big masses match the reference's; Flicker alternates the original and the mirror view in place.
For actual numbers, the value checker reads any point of a photo on the 1–10 value scale. The value range compressor squeezes a photo's values into the range your paints can reach.
Further reading
The page below goes further into the ideas the Black Mirror (Claude Glass) works with, and opens on the site named beside it.
- Value, notan and the art of seeing big masses (ArtisticMasterclass.com)
Tips from a working artist
From Sujith Puthran, artist and founder: what I actually do at the easel.
Judge the big masses at high strength, then lower it only to see which halftones survive.
From life, stand with your back to the view and move the phone until the masses fall into place.
Try the physical model at 40–70% adaptation for the look painters described.
Check your own painting in the mirror every session, not only at the end.
Why your picture stays on your device
The Black Mirror is a program your browser downloads and runs by itself. When you choose a picture, the browser reads it from your files and does the work on your own phone or computer; no copy is sent to this site or to anyone else, and what you export is saved straight from the browser. That is why there is nothing to upload and nothing to sign up for. The privacy page says what the site does collect.
Questions about the Black Mirror (Claude Glass)
The 5 questions below compare the mirror with a desaturated photo and its two models, then cover the camera and the exported file.
The mirror
How is this different from desaturating a photo?
The black mirror is different from desaturating a photo: desaturation removes colour and keeps every value and detail. The black mirror darkens the scene so halftones merge into their masses, keeps a little colour, and can bend the view. It answers whether the design reads, not what the values are.
Why does the physical model look so dark at 0% adaptation?
The physical model looks so dark at 0% adaptation because black glass reflects only about 4% of the light: the scene seen 4.5 stops darker. The eye adapts when you look into a real glass, which is what the adaptation slider adds back.
Why does the painterly model darken the corners?
Convex curvature in the painterly model adds a dark rim, like the edge of the glass. The physical model has none: its brightness varies only with the Fresnel reflectance.
Camera and files
Does the camera view upload anything?
No, the camera view does not upload anything. The video is drawn and processed in your browser, and nothing is recorded unless you freeze a frame and download it.
Is the exported PNG full resolution?
Yes for photos: the preview works on a copy scaled to 1,600 pixels, and the export runs the mirror again at full size. A frozen camera frame is saved at the camera's resolution.
- What a Claude glass was: en.wikipedia.org/wiki/Claude_glass · checked 2026-09-29
- What a Claude glass was: collections.vam.ac.uk/item/O78676/claude-glass-unknown · checked 2026-09-29
Look at a reference or the view itself in the glass, and paint the version whose masses still read.
Free · No sign-up · Runs in your browser