Dark Flash PhotographyDilip KrishnanRob Fergus

Dept. of Computer Science Courant Institute, New York University

Taking Photos in Low Light LevelsTaking Photos in Low Light Levels

Taking Photos in Low Light LevelsWant to avoid sensor noise & camera shake Possible solutions:

Drawbacks:Can be used in conjunction with our approachOur paper addresses these issues

Dark flash is ~200 times dimmer than conventionalGround truthReconstruction2. Ambient imageDark Flash Photography1. Dark Flash image

Key ChallengesHow to add light to the scene without it being perceived by people.

How to obtain an image to true colors.

Digital Camera SensorTwo SensorsThe Human Retina

Human Retina Spectral Sensitivity

Human Retina Sensitivity Log ScaleNote: Steep falloff outside 400-700nm range

Digital Camera SensorTwo SensorsThe Human Retina

Camera Spectral SensitivitySpectral 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

Camera Spectral SensitivityHuman Response Curve (not to scale)Comparison with human sensitivity curve

Cameras response is significantly broader

Possible to add light without being easy perceived

Dark Flash Emission SpectrumCamera Spectral Sensitivity

Dark Flash Emission

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

Comparison to Visible FlashBoth flashes will fire simultaneouslyBoth 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 day @ 1m

Equivalent to being outside for 1/100th second

Dark FlashDark Light @ 1m for ~1 secHazard Factor

Human Spectral Sensitivity FunctionVos 1978Shuman()

Log Human Spectral Sensitivity FunctionVos 1978log Shuman()

Spectral response curves for DSLR with IR-block sensor filter removedToo much IR sensitivity for our application add partial IR block filter to lensCamera 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

We are all familiar with the scenario: youre sitting in a bar or restaurant with friends and you want to take a picture, but light levels are low. *We are all familiar with the scenario: youre sitting in a bar or restaurant with friends and you want to take a picture, but light levels are low. *We will look at the spectral sensitivities of 2 types of sensor: the human eye and DSLR camera. First, well start with the eye.*We will look at the spectral sensitivities of 2 types of sensor: the human eye and DSLR camera. First, well start with the eye.*This is what our modified camera with flash looks like. Now I will do a demonstration of the dark flash. *This is what our modified camera with flash looks like. Now I will do a demonstration of the dark flash. *Physiologists have determined the sensitivity curves of the two types of receptors. Their response off rapidly outside the visible range 400nm to 700 nm. The blue curve refers to the scotopic gray-level cells and the red curves are the combined photopic cone cell sensitivities. The dashed black lines are at the 400nm and 700nm wavelengths and they mark the limit of the visible spectrum.*We can see this falloff more clearly if we view the log version of these functions. We can take advantage of this lack of sensitivity outside the 400-700nm range. If we were to illuminate the scene with light just outside this range, it would be barely noticeable to the eye.*