Microlens

From DpWiki

Introduction

Many modern image sensors make use of tiny microlenses mounted above each and every pixel. By focusing light into areas of the imager that are actually sensitive to light, these microlenses can dramatically increase the performance of the camera. In addition to the direct benefits, microlenses also provide manufacturers with increased flexibility in the designs of their sensors.

The Problem

Fig 1. - Without microlenses (top), a large percentage of the light projected on a pixel hits areas of the sensor that are not sensitive to light. Microlenses (bottom) help to focus as much light as possible into the photosensitive areas of the pixel. This can result in a dramatic increase in the sensitivity of the image sensor.

In most common forms of modern image sensors, only part of each photosite is actually sensitive to light. These inactive areas of the sensor are used for supporting electronics that allow image data to be amplified, read off or otherwise processed. While these components perform important tasks, the light falling on them can not contribute to the final image. As such, the lower the fill rate (percentage of each pixel used to capture light) the lower the sensitivity of the camera.

In addition to these direct benefits, microlenses also provide engineers with the leeway to integrate additional features into their designs. As microlenses can offset the disadvantages of low fill rates, this allows these designers to use more of a photosite's available area for peripheral functionality.

Solution - Microlenses

To help resolve this problem, camera manufacturers have taken to placing an aray of tiny lenses directly above the image sensor. The array is bonded to the surface of the imager such that each microlens lines up with a single pixel (see Fig. 1, bottom). These lenses then focus the light falling on the sensor into the photosensitive regions.

Image sensors not equipped with microlenses throw away a lot of the energy projected by the lens (Fig. 1, top). As microlenses help to ensure that more of that light can actually be captured, they provide dramatic improvements in the overall sensitivity of the camera. As such, many current sensor technologies would not be practical without the use of microlenses.

Side Effects

Light Falloff

The effectiveness of modern microlenses often falls off dramatically as the angle of incidence increases. This can result in noticable light falloff in the extreme edges of the frame when used with certain lens designs. While most digital sensors have a similar sensitivity to off-axis light on their own, the presence of microlenses generally agrivates this problem further.

It should be noted that this issue is strongly dependant on the design of the specific microlenses in use. In some cases it may even be possible to design microlenses in such a way as to counteract the inherent light fall off of the underlying sensor. As such, this is not necessarilly an inherent fault of the technology but a limitation of current designs.

See Also

• Bayer Filter Array - Alows inherently monochromatic image sensors to capture and interpret colour information.
• Anti-Aliasing Filter - A special filter that adds a small amount of blur to captured images in order to prevent aliasing artifacts.
• Electronic Shutter - Used in some types of image sensors, electronic shutters eliminate some of the limitations imposed by mechanical shutter assemblies.