The following article describes tips for digital microscope photography. It was written by Yong Sun of Polaroid Corporation and originally appeared in the July, 1998 edition of Advanced Materials and Processes magazine. It is reproduced here with the kind permission of AM&P Editor, Margaret Hunt. While it is biased toward Polaroid's DMC camera there are some universally applicable tips.
|The minimum configuration required to effectively capture and control
digital images for digital microscope cameras (such as Polaroid's DMC) requires
a Pentium-level processor crunching data at 120 MHz. Memory needs include
20 MB of standard RAM, 2 MB of video or VRAM, and 540 kB of cache memory.
For local storage, a one gigabyte hard drive and a 15-inch monitor are essential.
Microsoft Windows 95 or Windows NT operating system and a TWAIN-compliant image processing application for PC-based systems are also required. Apple users will need a Power PC processor and an Apple System software version 7.5 or later.
However, the more computer you have the better. Also, RAM -- both standard and video -- can often do as much to speed system processing as the processor itself, and often for hundreds of dollars less.
Like any camera, the quality of the image depends -- to a large degree -- on the quality of the lens. However, in this case, since the camera lens is actually a CCD camera, the significant lens is in the microscope. Although software may enhance digital images, it cannot always make up for a mediocre image from a low-quality microscope optics.
For most materials science applications, standard magnifications range from 50X to 200X. Many stereo microcopes commonly have standard magnifications up to 100X. For higher levels of magnification, a more expensive microscope is needed. In addition to superior optics, some scopes offer magnification up to 500X or even 1000X.
Mounting the camera
The camera mounting is key when it comes to quality and size of the digital image. The DMC works best when coupled to the microscope with a standard 0.63X C- mount. With a C- mount, an object in view at the focal plane will produce an image in focus at the photo plane, or in the case of the DMC, at the CCD sensor.
If you are tempted to experiment with other mounts, know from the start that the results will rarely, if ever, be worth the time and effort. For example, an F-mount is designed for a 35-mm focal length. This complicates the interface between the camera and the microscope lens, and may result incapture of an unacceptably small area of the sample.
Also, note that some C-mounts are better than others. A 0.63X C-mount or larger is highly recommended, because the DMC has a large, 12.15 millimeter CCD sensor chip. With a smaller number C-mount, you will not be able to take full advantage of the available viewing area made possible by the large chip. In other words, you will not be able to view the entire available image.
The C-mount offers other advantages as well. It allows the DMC to be mounted in any direction, making it suitable even for inverted microscopes. It also allows the DMC to be moved easily from one microscope to another, enabling one camera to service several microscopes.
Finally, the DMC may be used with any standard C-mount lenses for other applications, such as copystand work.
Most light sources are built into the microscope and generally are either reflected or direct light. These systems can be very sophisticated and can include a "dimmer switch" type adjustment, a built-in light temperature feature, color correction filters, and more.
Unfortunately, these "enhancement" features sometimes work in the wrong direction.
For example, users frequently reduce their microscope's light source in an effort to make it "easier on the eyes." This may be acceptable for viewing, but not for photography. The bottom line is that acceptable photographic images need bright light.
In medical or forensic applications, users sometimes misuse the lighting intensity for exposure adjustment. Again, this is not recommended. Rather, turn up the light to a temperature of 3200K or 5500K, and either add color-correction filters or adjust the colors in the DMC settings screen.
In semiconductor applications, the optimum temperature is again 3200K or 5500K. If glare or reflection is a problem, use a polarizing density filter.
For auxiliary or external lighting systems, fiber optic lights or halogen lights can be very effective in low- magnification applications below 100(X). These systems are generally quite affordable, effective, and easy to adjust and control.
Before acquiring the image, it is essential that the focus through the microscope eyepiece and on the preview screen be the same.
With a focusable C-mount, start by viewing the image under the scope in focus with all eyepiece settings.
Then adjust the image in the preview screen by focusing the C- mount until it is as sharp and crisp as possible. In the end, good lighting, the right microscope optics, and sharp focus are the keys to making the best acquisition in terms of clarity, color, and contrast.
Because digital microscopy is a system, software is vital to maximizing the technology. For example, the preview mode allows you to manipulate the sample, identify the target area, adjust the focus, and preview the final image on the screen before you actually capture the image. Also, DMC sofhvare includes a white balance function. Once the computer knows "what's white," all other colors will be fairly accurate. Another useful feature is a calibration marker, which indicates the exact magnification at which the image was captured. Whether the image is enlarged or reduced, the marker remains "to scale."
Saving the image
A common challenge with digital imaging is not just in how the image is photographed, but also in how it is saved and managed. Many users "under-acquire" images in an effort to keep file sizes to a minimum. Unfortunately, this approach can lead to problems, especially when the need arises to output the image in print, or to "blow up" only a portion of the image.
When deciding on the size of the image, always think ahead to the final size you will ultimately need, and then acquire the image at the optimum resolution.
If you know that the image will never be used for anything other than an on-screen application, then 72 dpi is acceptable. But if the image may one day be printed, never consider less than 133 dpi, with 300 dpi even better.
Like any new product or technology, experience is the key to getting the most out of any digital microscope camera.
The more you work with it, the more familiar you will become with its features and capabilities. And because the images are digital and not on expensive film, it costs nothing to experiment.
For more information: Yong Sun is a senior marketing manager with Polaroid Corp., 575 Technology Square, Cambridge, MA 02139; tel: 781-386-6826; Intemet: www.polaroid.com