dark flash photography

Dark Flash Photography

Dilip KrishnanRob Fergus

Dept. of Computer ScienceCourant Institute,

New York University

Taking Photos in Low Light Levels

Taking Photos in Low Light Levels

Taking Photos in Low Light LevelsWant 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 conjunctionwith 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 falloff outside400-700nm

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-blocksensor filter 2. Adding lens filter to remove IR >800nm

Some sensitivityin UV & IR just outside visible

Camera Spectral Sensitivity

Camera Spectral Sensitivity

Human Response Curve (not to scale)

• Comparison with human sensitivity curve

• Camera’s response is significantly broader

• Possible to addlight without being easy perceived

Dark Flash Emission Spectrum

CameraSpectralSensitivity

Dark FlashEmission

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 Flashesper day @ 1m

• Equivalent to being outside for 1/100th second

Dark Flash

Dark Light @ 1m for

~1 sec

Hazard Factor

Human Spectral Sensitivity Function

Vos 1978

350 400 450 500 550 600 650 700 750 8000

200

400

600

800

1000

1200

1400

1600

1800

Wavelength (nm)

Luminous efficacy (lumens/watt)

Scotopic (rod - dark adjusted)Photopic (cones - bright light)

Shuman(λ)

Log Human Spectral Sensitivity Function

Vos 1978

350 400 450 500 550 600 650 700 750 800-4

-3

-2

-1

0

1

2

3

4

Wavelength (nm)

Log Luminous efficacy (lumens/watt)

Scotopic (rods - dark adjusted)Photopic (cones - bright light)

log Shuman(λ)

• Spectral response curves for DSLR with IR-blocksensor filter removed

• Too much IRsensitivity forour application

add partialIR block filterto lens

Camera Spectral Sensitivity

• Spectral response curves for DSLR with IR-blocksensor filter removed& IR lens filter

• Some sensitivityin UV & IRjust outsidevisible

Camera Spectral Sensitivity