Dark Flash Photography Dilip Krishnan Rob Fergus Dept

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

Taking Photos in Low Light Levels

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

Dark Flash Photography • Dark flash is ~200 times dimmer than conventional 1. Dark Flash image 2. Ambient image Reconstruction Ground truth

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.

Two Sensors The Human Retina Digital Camera Sensor

Human Retina Spectral Sensitivity

Human Retina Sensitivity – Log Scale Note: Steep fall off outside 400 -700 nm range

Two Sensors The Human Retina Digital Camera Sensor

Camera Spectral Sensitivity • Spectral response curves for an unmodified DSLR

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

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

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 ~220 x 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 day @ 1 m • Equivalent to being outside for 1/100 th second Dark Flash Hazard Factor Dark Light @ 1 m for ~1 sec

Human Spectral Sensitivity Function Shuman(λ) Luminous efficacy (lumens/watt) 1800 1600 Scotopic (rod - dark adjusted) 1400 Photopic (cones - bright light) 1200 1000 800 600 400 200 0 350 400 450 500 550 600 650 Wavelength (nm) 700 750 800 Vos 1978

Log Human Spectral Sensitivity Function Log Luminous efficacy (lumens/watt) 4 3 log Shuman(λ) 2 1 0 -1 Scotopic (rods - dark adjusted) -2 Photopic (cones - bright light) -3 -4 350 400 450 500 550 600 650 Wavelength (nm) 700 750 800 Vos 1978

Camera Spectral Sensitivity • 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