High Speed Microscope

The Limit Is Exposure, Not Frame Rate

A microscope used on a moving part is limited by how short the exposure can be while still producing a usable image. The frame rate figure describes how often an image can be read out; the exposure figure describes how long the sensor collects, and only the second determines whether the image is blurred.

A camera that reads out at a high rate with a long exposure produces a fast sequence of blurred images. The two specifications are often quoted together and confused.

Motion Blur Is Arithmetic

The blur in the image equals the distance the part travels during the exposure. For a part moving at a known speed, the maximum exposure that keeps blur below one pixel follows directly.

The relevant distance is the object-space distance, not the image-space distance, so the magnification enters the calculation. Higher magnification makes the same part motion produce more pixels of blur, which is why high-magnification inspection of moving parts is the hardest case.

Doing that arithmetic before the camera is chosen converts a vague requirement for speed into an exposure time that the lighting can be designed against.

The Illumination Budget Follows

A short exposure demands light. The product of intensity and exposure time has to produce enough signal, so cutting the exposure by a factor requires roughly that factor more intensity.

This is where a continuous source usually fails and a strobed source becomes necessary. A strobe concentrates the output into a pulse synchronised with the trigger, delivering intensity that a continuous source of the same average power cannot.

The consequence for the station design is that the trigger, the controller and the light have to be specified together with the camera rather than after it.

Readout and Data Path

Frame rate is bounded by readout and by the interface. A sensor that can expose quickly may still be limited by how fast its data can leave, and an interface that is adequate for a single camera may not be for two.

The practical figure is the sustained rate with the region of interest and the bit depth the application uses. Brochure frame rates are usually quoted at the smallest region and the lowest bit depth.

Depth of Field Becomes a Time Problem

At high magnification the depth of field is small, so a moving part passes through focus rather than staying in it. The question is no longer whether the image is sharp but whether it is sharp at the moment of capture.

An autofocus system cannot follow fast motion, because it needs several frames to establish focus. The reliable alternative is to trigger capture at a known position, using a position sensor or an encoder, so that the part is in focus by construction.

This changes the station design from a camera problem into a mechanism problem, which is usually the cheaper place to solve it.

What to Specify

Specify the part speed, the maximum blur expressed in object-space micrometres, the feature size, and the field. From those the exposure, the magnification and the illumination requirement follow.

Then specify the trigger source and the position accuracy at which the capture occurs, because those determine whether the exposure can be short for a reason.

A specification that asks for a frame rate without these four cannot be answered, and a supplier who answers it anyway has answered a different question.

Frequently asked questions

Does a higher frame rate reduce motion blur?

No. Blur is set by the exposure time, not by the frame rate. A high frame rate with a long exposure produces a fast sequence of blurred images.

How do I calculate the maximum exposure for a moving part?

Divide the allowed blur in object space by the part speed. Convert the allowed blur from pixels using the object-space size of one pixel, which depends on the magnification.

Why does high magnification make motion blur worse?

Because the same physical movement covers more pixels at higher magnification. The blur is a distance on the part, and the magnification determines how many pixels represent it.

Do I need a strobe light?

Usually, when the required exposure is too short for a continuous source to deliver enough light. A strobe concentrates the output into a pulse synchronised with the trigger.

Can autofocus keep up with a moving part?

Not at production speeds, because autofocus needs several frames to establish focus. The reliable approach is to trigger capture at a known position using an encoder or a position sensor.

What should I state in a high-speed requirement?

Part speed, the maximum allowed blur in object-space micrometres, the feature size and the field. The exposure, magnification and lighting requirement all follow from those four.