WP26 LUMILEDS | Alldatasheet
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WP26 Flash for Selfies White Paper 20161202 ©2016 Lumileds Holding B.V. All rights reserved. market share over Digital Still Cameras due to its convenience. Figure 1. Expected market share of different image sensor resolution. that camera flash is starting to be more readily implemented for front cameras also. certain illuminance requirements and shorter flash pulses that are recommended.
WP26 Flash for Selfies White Paper 20161202 ©2016 Lumileds Holding B.V. All rights reserved. 3 Typically, the total SNR is determined by the level of shot noise. Shot noise can be improved by increasing the number of photons hitting each pixel. The following parameters are crucial:
- Pixel size (the number of photons collected is proportional to the pixel area)
- Sensor construction—the newer backside illumination (BSI) sensors use the complete available area by having the light sensitive area at the top of the sensor and the metallization layers below.
- The so-called F-number (F#) of the camera lens which is the lens focal distance f divided by the lens diameter. The signal will decrease inversely proportional with the square of F#, as the amount of light collected and illuminated spot size at sensor position will increase with the square of F#.
- Exposure time—increase of this will increase the SNR. The maximum exposure time, however, is limited by the maximum time the camera can be held still without image blur due to handshaking (without tripod). A general rule of thumb advises to keep the shutter speed [expressed in seconds] smaller than 1/focal length [in mm-1]. For phone cameras, this rule of thumb’s maximum exposure time falls in the 30ms. The time can be prolonged (factor 2 and more) by image stabilization/ shake reduction technologies.
- Transmission of the complete camera lens system and filters in the camera’s light path. Performing the calculations of sensor signal to expect for F# of 2.4, integration time of 50ms and pixel size of 1.4um, an illuminance of 100lux (or 5lux.s) on scene produces an SNR on the picture of about 36dB—which is considered a good SNR. Note that with the increasing resolution race, the pixel size reduces further, regardless of resolution. To compensate the increasing light thirst of these smaller pixels (1.1um=1.6 more light needed compared to 1.4um), the F# goes down, too: numbers of 2.2 now not go hand in hand, the illuminance of 100lux should proportionally increase. In the case of a dim environment, the camera flash needs to supply the targeted illuminance for the requested time. The illuminance must be realized over the complete field of view (FOV) of the camera with only gradual decrease towards the borders of the scene (keeping the maximum illuminance for the center where the subject is in general located at). While the necessary illuminance for the camera is the same, independent of the distance of the camera to the scene (as the scene is imaged), the flux requirement will increase with the square of the distance between flash and scene in order to achieve the average illuminance of the scene. The maximum user distance will define how much flux is needed from the flash. For instance, for the front facing camera, the user distance will be about arm length, or about 0.5m. (For the rear camera, the user distance will be 1m or more. With a target distance of 1m, the rear camera flash will have to deliver already 4 times as much light.) Due to the always decreasing thickness of smartphone builds, the FOV increases. For front cameras, FOV around 80° are common. For these larger FOV, the camera flash optics must make the beam slightly broader than a bare LED’s Lambertian radiation pattern. The on-axis illuminance per LED flux will be at best close to 0.3lux/lm in typical smartphone geometries. Therefore, to achieve a target of 100lux at a distance of 0.5m, the necessary flux out of the LED will be around 80lm. Glare A particular point of attention is glare sensation caused by the flash. As the person taking the picture needs to closely look back at the phone and screen to check the settings, the torch and flash light can cause discomfort in some cases. The position of the flash and time can have an effect on glare impression: 1. Use indirect flash light: As in the photographer’s studio, a flash light pointing to studio walls and the ceiling to create an “ambient” lighting rather than a direct flash illumination. For the front facing camera, the situation around the subject is not as controlled as in the studio, not to mention that ceilings and walls may not be present. 2. Use shorter flash pulses: In a paper from Ronald Gibbons and Christopher Edwards, “A Review of Disability and Discomfort Glare Research and Future Direction,” the effects of glare in traffic situations was summarized. The article cites Lehnert and his investigation that a shorter flash light as generated by a vehicle bounce can result in lower glare sensation than longer duration of the same illuminance. The equation given is: 10.61. 0.0318 0.0064
t pulse is the duration of the light pulse[s]. Table 1. DeBoer scale.
1 Unbearable
3 Disturbing
5 Just admissible
7 Satisfactory
9 Unnoticeable
higher illuminance, the time can be shorter. during the short flash pulse. Figure 4. Illuminance and pulse length for constant light dose vs. deBoer scale. the moment of exposure are shifted in time with respect to each other.
the same time. Last, by reducing the exposure time, motion blur in pictures with moving objects is reduced. Figure 5. Rolling shutter time sequence with integration times and readout. Table 2. Side by side comparison of long vs. short flash pulse.
- 67ms for 15fps; handshake 30ms with stabilization.
- Measured pulse width of smart phone about 4x exposure time.
WP26 Flash for Selfies White Paper 20161202 ©2016 Lumileds Holding B.V. All rights reserved. 7 For the implementation in a phone, the synchronization signal must come from the camera to precisely fit the moment of flashing with the frame-blanking period. As with any picture taken with flash, pulse amplitude and/or pulse length should be adjusted to optimize exposure level. The flash sequence could be 1) probably a very low level torch mode to tune auto focus, followed by a 2) pre-flash of similar energy as the main flash to determine best exposure with flash conditions to, finally, flash 3) the main, short flash within frame blanking. Summary Camera flash units are increasingly built into the front facing cameras of smartphones. For presently available sensors of 1.4 micron pixel size, a flash delivering 100lux illuminance (light dose 5lux.s) should produce a picture with a good signal-to-noise ratio of 36dB. With 0.5m distance between the camera and the subject (for the taking of a “selfie”), the necessary flux from the flash is about 80lm for the typical broad FOV of 80° to 85°. To reduce glare when taking “selfies,” the pulse length should be reduced (≤25ms) to take advantage of the time-dependence of the glare sensation. Flash pulses this short should be timed in the frame blanking of a rolling shutter. The LED used for front facing flash needs to support high currents (order of 1A) to achieve the necessary light dose for a well-illuminated picture within the short flash pulse.
©2016 Lumileds Holding B.V. All rights reserved. LUXEON is a registered trademark of the Lumileds Holding B.V. in the United States and other countries. lumileds.com Neither Lumileds Holding B.V. nor its affiliates shall be liable for any kind of loss of data or any other damages, direct, indirect or consequential, resulting from the use of the provided information and data. Although Lumileds Holding B.V. and/or its affiliates have attempted to provide the most accurate information and data, the materials and services information and data are provided “as is,” and neither Lumileds Holding B.V. nor its affiliates warrants or guarantees the contents and correctness of the provided information and data. Lumileds Holding B.V. and its affiliates reserve the right to make changes without notice. You as user agree to this disclaimer and user agreement with the download or use of the provided materials, information and data. WP26 Flash for Selfies White Paper 20161202 About Lumileds Lumileds is the global leader in light engine technology. The company develops, manufactures and distributes groundbreaking LEDs and automotive lighting products that shatter the status quo and help customers gain and maintain a competitive edge. With a rich history of industry “firsts,” Lumileds is uniquely positioned to deliver lighting advancements well into the future by maintaining an unwavering focus on quality, innovation and reliability. To learn more about our portfolio of light engines, visit lumileds.com.