Throwback Thursday: Aligning a sensor with a slide projector

Adimec celebrates its 25th Anniversary this year. During the past 25 years we have gathered a large amount of knowledge about cameras.

The tooling for building cameras has changed over time. In our Throwback Thursday blog series we will give you a look into the history of camera manufacturing at Adimec.

Today’s topic is a sensor alignment and linearity measurement tool. The tool consists of a slide projector, a mechanical stage to hold and rotate the camera, and a huge box with control electronics. This tooling was used for the TMX5 CCD camera. It was used until 1996.

The tool aligned the lensmount with the sensor and measured the linearity of the sensor.

Lensmount to sensor alignment

It is important for the lensmount to be aligned to the sensor accurately as this allows the center of the lens to be aligned with the center of the sensor. A center to center alignment of lens and sensor reduces optical artifacts in the image.

For the alignment, a crosshair slide was put in front of the projector light bulb. With a lens, the crosshair was projected on the camera. The camera also had the option to display a digital crosshair. The right alignment could be achieved by matching the two crosshairs on the image.

The mechanical stage was able to rotate the camera with respect to the lens. For the alignment to be correct, rotating the camera by 180° should not influence the position of the crosshair.

The alignment had to be done by manually adjusting screws on the lensmount. It could achieve an accuracy of 5 pixels in the x and y direction. With a pixel size of approximately 8 by 17 micron this is an accuracy of 40 to 85 micron. This is already very much comparable with what is possible now. However, in this system the lensmount had to be aligned manually with the sensor while nowadays the sensor is automatically aligned with the camera housing. A lensmount can be fixed to the camera at a later step. This of course works much more efficiently and also allows for choosing a different lensmount with a different thread.

Linearity measurement

For all cameras that we produce we perform several measurements that together make the specifications. With those measurements we know that the delivered cameras conform to the specifications. One of these measurements is linearity. Linearity is an important specification as it basically tells you in which signal range the camera behaves linearly (1 photon produces 1 electron) with the amount of incoming light. Outside the linear signal range it is often difficult to draw any conclusions from the measurements.

For the TMX5 this linearity was thus measured with the help of a slide projector and slides that contained neutral density (ND) filters. By selecting everytime a differend slide with a different ND in front of the light bulb and measuring the signal intensity of the camera each time, a light intensity versus signal amplitude graph could be constructed. From this graph, the linearity was calculated.