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Flash for “Seles”

An overview of illuminance requirements

and why shorter ash pulses are the

preferred solution for front ash

Introduction

Smartphones are ubiquitous in everybody’s daily lives, a trend that shows no sign

of slowing. A key component of the smartphone is the camera, which has gained

market share over Digital Still Cameras due to its convenience.

Figure 1. Expected market share of dierent image sensor resolution.

As the demand for smartphone cameras increases, sensor makers are continuously

working to improve the resolution, as demonstrated by Figure 1. And while 20MPix

capability gained in importance for the main camera of the smartphone, the

resolution race has begun for the front camera. With the rise in popularity of “seles”

and the 5 to 8 Mpix resolution for the front camera (see Figure 2), it is not surprising

that camera ash is starting to be more readily implemented for front cameras also.

However, to make a successful front ash that captures an ideal “sele,” there are

certain illuminance requirements and shorter ash pulses that are recommended.

Figure 2. Expected market share of dierent sensor resolution in smartphone camera.

Illuminance Requirements

In low ambient lighting conditions, the camera ash needs to supply the illumination for the scene of the picture. The camera’s

lens makes an image of the scene with the light reected by the dierent scene objects.

Figure 3. Light path ash to camera.

Imaging means that each pixel has a spatial correlation to a dened area of a scene. Each pixel thereby receives reected light

from that area in the scene, after passing through the lens and dierent lters. This light is absorbed by the sensor pixel and

converted into electrons and transferred as ‘charge packages’ per exposure time for further (electronic) processing.

The pixels signal will thereby be proportional to the light dose per area—which is illuminance, multiplied by the integration or

exposure time:

Signal

Sensor

a Illuminance

Scene

· Pixel area · Exposure time

To dene the necessary light dose, the rst decision to make is the targeted signal-to-noise ratio (SNR). Noise contributors include

shot noise (proportional to 1⁄

√

number of photons), electronic noise (amplier), A/D noise (number of bit levels) and dark noise in

the camera path.