A Guide to Choosing the Right Digital Microscope Camera for the Application

Pixel Size

A key criterion for choosing the right camera is the amount of light at the microscope. The less light available, the greater the light sensitivity of the camera needs to be. Generally speaking, a pixel size of 6.45 µm or larger offers excellent light sensitivity for crystal-clear, low-noise images. However, this comes at a trade-off because the larger the physical pixel size, the less pixels can be fit on the sensor, reducing resolution.

Sensor Size

The sensor sizes determine the real-estate in which pixels are placed in an array. Most scientific-grade cameras incorporate large sensors, for example, 2/3” – 1”, because pixel size and pixel quantity are both maximized. However, cameras used for documenting conventional bright-field images may use an image sensor smaller than 1/2” due to their inherent smaller pixel sizes and smaller quantity of pixels. Regardless, the correct magnification c-mount adapter must be chosen based on the sensor size to avoid vignetting or compromised field-of-view.

Resolution

Driven by the mega-pixel hungry consumer market, which users know from photography, digital resolution, or quantity of pixels, is easily misperceived as image quality. Truth is, micro-imaging applications require less pixels than macro-imaging. If you work mostly at very high magnifications, the optical system is limited to about 3–5 megapixels that can be transferred to the sensor of a camera. Setting the camera to a high resolution of, say, 12 megapixels would produce a larger image, but you would not gain any additional information. If you use the microscope at low magnification on the other hand, then you definitely need a high resolution digital camera to capture all the details that your microscope can deliver. To learn more about the relation between resolution and image quality with microscope cameras read the short explanation on Leica Science Lab. http://www.leica-microsystems.com/science-lab/digital-cameras/

CCD vs. CMOS

While many CMOS sensors deliver adequate performance for simple, routine digital image documentation or basic image measurement, more demanding applications require higher performance. Although CMOS cameras are cheaper to mass-produce and are rapidly evolving, they still produce a “rolling shutter” effect, which reduces fluidity when moving a sample across the stage. Further, key parameters such as Dynamic Range, Signal-to-Noise and Color Fidelity are often compromised as compared to their CCD counterparts.

Color or Gray-Scale?

While there is often a stringent requirement to reproduce colors to the greatest degree of accuracy for documentation or color image-analysis, the Bayer Mask permanently mounted in front of the image sensor to interpolate color, comes at the high cost of reduced efficiency under low-light situations. This is why, for example, low-signal fluorescent images are acquired with a high-sensitivity monochrome camera, then pseudo-colored to their particular emission characteristics.

Dynamic Range

This is the camera’s ability to segment the digital image into shades of gray or color, accounting for noise. For example, a camera with a dynamic range of 500:1 can divide photons of light into 500 divisions from dark to light. Cameras with larger dynamic ranges can better define subtle changes in color, and are highly recommended when using Image-Analysis to measure these gradations. While often misperceived as Dynamic Range, a camera’s bit depth (i.e. 8-bit, 12-bit, etc.) correlates to the maximum number of gradations that can be potentially measured.

Fast Image Processing

Image processing speed is a criterion for applications requiring not only fast and highly accurate visualization of microstructures in color, but also quick archiving and analysis. Microscope cameras from Leica Microsystems digitize the image information from the CCD or CMOS chip in the camera head itself: thanks to the FPGA (field programmable gate array) function, data do not have to be sent to the computer first. Color interpolation, image sharpening and shading corrections are performed directly in the camera hardware without impairing the live image speed. This results in low-noise images and ultra-fast image processing.

Fast and Fluid Live Imaging

The human eye can interpret motion at approximately 30 frames per second, so the ideal camera should match. High speed, or high refresh-rate, allows fast and fluid motion of the sample underneath the microscope, and aids tremendously when focusing.

Full HD Imaging

Many cameras offered by Leica Microsystems are capable of producing a stunning full HD image. This means that a customer can simply connect the camera, via a single, supplied HDMI cable, to any HD monitor and view a breathtaking HD image at 30 fps within seconds after installation. What’s more, a PC is not required for image or video capture, as data is stored on the included SD card. Image and video acquisition are optimized and triggered via remote control. Often used in a discussion or workshop, you can instantly share what is seen under the microscope without the complication of a PC.

Fast Data Transfer

Certain applications, such as acquiring a series of z-stacks, or panoramic image creation with an integrated motorized stage, require data to be transferred from the camera to hard-disk at high speed. Eclipsing the throughput capabilities of former interfaces, such as USB 2.0 or FireWire, USB 3.0 is evolving into the interface of choice for high-speed data transfer. Further, no internal interface cards are required, allowing plug-and-play with notebook PCs.

Software Integration

With all of the potentially daunting number of variables offered by the microscope and camera, such as magnification, contrasting technique, illumination type and level, exposure time, electronic gain, etc., software is responsible for harmonizing all components and making the optimization process simple and repeatable. For example, Leica Application Suite allows the operator to record the optimal imaging parameters automatically, within the image meta-data.  And with one click, the hardware and camera settings can be reproduced for use with subsequent image acquisition. What’s more, modern digital imaging algorithms, such as High Dynamic Range, or HDR, acquire a series of images at varying exposure times, then automatically build a composite image based on the optimally exposed pixels, setting a new precedent for previously unattainable levels of contrast. Also, operators can now simply move the focus knob and a perfectly in-focus image is created based upon the sharpest, most in-focus, pixels.

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