Dark Flash Photography Dilip Krishnan Rob Fergus Dept. of Computer Science Courant Institute, New York University

Taking Photos in Low Light Levels

Want to avoid sensor noise & camera shake Possible solutions: 1.Large Aperture Lens Expensive, Depth-of Field issues Drawbacks: 2.Image stabilization Only gives a few extra stops of exposure 3.Camera flash Active method, Disturbs Scene Can be used in conjunction with our approach Our paper addresses these issues

Dark flash is ~200 times dimmer than conventional Ground truthReconstruction2. Ambient image Dark Flash Photography 1. Dark Flash image

Key Challenges 1.How to add light to the scene without it being perceived by people. 2.How to obtain an image to true colors.

Digital Camera Sensor Two Sensors The Human Retina

Human Retina Spectral Sensitivity

Human Retina Sensitivity – Log Scale Note: Steep fall off outside nm range

Digital Camera Sensor Two Sensors The Human Retina

Camera Spectral Sensitivity Spectral response curves for an unmodified DSLR

Modify camera by: 1. Remove IR-block sensor filter 2. Adding lens filter to remove IR >800nm  Some sensitivity in UV & IR just outside visible Camera Spectral Sensitivity

Human Response Curve (not to scale) Comparison with human sensitivity curve Camera’s response is significantly broader Possible to add light without being easy perceived

Dark Flash Emission Spectrum Camera Spectral Sensitivity Dark Flash Emission

Demonstration of Dark Flash Please look directly at the demonstration, NOT at the projection screens

Comparison to Visible Flash Both flashes will fire simultaneously Both give comparable camera exposure Dark flash is ~220x dimmer than visible flash

Flash Safety Eye is most sensitive part of the body to UV Government TLV tables specify Hazard Factor & safe limits  Safety limit is 115,000 Flashes per 1m Equivalent to being outside for 1/100 th second Dark Flash Dark 1m for ~1 sec Hazard Factor

Human Spectral Sensitivity Function Vos Wavelength (nm) Luminous efficacy (lumens/watt) Scotopic (rod - dark adjusted) Photopic (cones - bright light) S human (λ)

Log Human Spectral Sensitivity Function Vos Wavelength (nm) Log Luminous efficacy (lumens/watt) Scotopic (rods - dark adjusted) Photopic (cones - bright light) log S human (λ)

Spectral response curves for DSLR with IR-block sensor filter removed Too much IR sensitivity for our application  add partial IR block filter to lens Camera Spectral Sensitivity

Spectral response curves for DSLR with IR-block sensor filter removed & IR lens filter Some sensitivity in UV & IR just outside visible Camera Spectral Sensitivity