Digital cameras require some level of exposure time adjustment in order to control the levels of brightness, color saturation, and contrast in the final image. Three factors defining correct exposure in a charge-coupled device (CCD) camera are the wavelength-dependent visible light spectrum sensitivity, the intensity of illumination, and the length of time the CCD is exposed to light.

Current specimen exposure for the DN100 camera system is indicated on the Exposure Display indicator of the CAM submenu window as illustrated in Figure 1. The CAM submenu can be accessed directly on the display window by right-clicking on the lower portion of this window while no other menu is activated.

One of the most important variables in digital photomicrography is to assure the correct exposure to match the dynamic range and sensitivity of the camera system. The DN100 camera control unit is equipped with an exposure time control feature that allows the microscopist to capture exact and reproducible exposures of images observed in the eyepieces of the microscope. The camera control unit can produce exposures in a wide range of time intervals between one thousandth of a second (1/1000) and 60 seconds.

Figure 2 illustrates the color codes for exposure level states in the DN100 control software. Exposure is a function of several variables, including the input brightness of the optical image originating from the microscope, as well as the currently selected camera exposure mode, exposure time, and compensation or gain settings. An important feature of the DN100 camera control unit is that it can be configured to automatically adjust exposure time and camera gain in order to compensate for variations in specimen illumination. The DN100 has four basic exposure modes, which are described as follows:

• Program AE Mode - In this mode, the camera gain and the shutter speed change automatically to yield the correct exposure, even if specimen brightness fluctuates or changes. The shutter speed automatically compensates over a range from 1/1000 second to 1 second, and the camera gain can range between x1 to x4 for additional exposure control. Exposure compensation can be adjusted over a range between -2.0 EV to +2.0 EV in 0.3 EV steps.

• Shutter Speed Priority AE Mode - Camera gain is determined automatically to yield the correct exposure for the selected shutter speed. The shutter speed can be adjusted over a range of 1/1000 second to 4 seconds. Exposure compensation can be adjusted over a range from -2.0 EV to +2.0 EV in 0.3 EV steps.

• Manual Exposure Mode - This mode permits manual selection of the camera gain and the shutter speed. The shutter speed can be adjusted over a range from 1/1000 second to 60 seconds. Camera gain can also be adjusted over a range from x1 to x46.

• Focus Priority Exposure Mode - This is a useful mode for imaging a poorly illuminated specimen. Although the camera gain and shutter speed are determined automatically, the shutter speed is limited to a range that permits focusing. The shutter speed changes automatically over a range from 1/1000 second to 1/8 second while the camera gain automatically changes over a range from x1 to x46. Exposure compensation can be adjusted over a range from -2.0 EV to +2.0 EV in 0.3 EV steps.

A bracketed series of exposures captured using the DN100 camera system coupled to a Nikon Eclipse ME600 microscope operating in reflected differential interference contrast (DIC) mode are presented in Figure 3. The specimen is a wafer containing many copies of the MIPS R4400 microprocessor integrated circuit. Each image in the series was exposed for a time period that was 50 percent less than the preceding image. For example, image (a) in Figure 3 was exposed for one-half (1/2) second, while image (b) was exposed for one-quarter (1/4) second, and image (c) was exposed for one-eighth (1/8) second. The last three images (d-f) were exposed for one-sixteenth (1/16), one-thirtieth (1/30), and one-sixty-fourth (1/64) second, respectively. The image having the best overall exposure, color contrast, and saturation is image (d), which is positioned near the center of the bracket.

The DN100 exposure controls are accessible from a number of locations. Additional information on operating the DN100's exposure controls can be found in the following sections:

• Main Menu Features - A central feature of the DN100 network camera system main menu window is the convenient button toolbar, which provides instant access to a number of the most commonly used camera control functions.

• CAM Submenu Features - This important menu contains a number of features useful for measuring and calibrating image focus, controlling exposure mode and time, and adjusting camera sensitivity.

• Power On Setting Panel - Up to three custom environments for the DN100 camera control unit can be configured by the operator with this setting panel.

• Room 1 Network Client Interface - Providing a network-based remote access to the DN100's camera control unit, this virtual room allows a remote operator to download live microscope images from the camera control unit. Other features include an image annotation function and an electronic zoom function. An option is also available to allow the remote user to take limited control of the camera control unit.

In conclusion, the DN100 camera control unit is equipped with a wide spectrum of features that allow the user to accurately monitor effects of both the camera sensitivity and exposure characteristics on resulting digital images captured from the microscope. These features are very useful in producing sharp, crisp, and clear digital photomicrographs that display an excellent blend of color balance, saturation, and contrast.