Exposure Control and Camera Sensitivity
Digital cameras require some mechanism of exposure 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 visible light spectrum wavelength-dependent sensitivity, the intensity of illumination, and the length of time the CCD is exposed to light.
Current specimen exposure status for the Digital Sight camera system is indicated by the Exposure Level indicator of the CAM menu window as illustrated in Figure 1. When the Advn. tab of the CAM menu is selected, additional exposure information is displayed, including shutter speed, exposure mode, and other variables. The CAM menu is called up either by pressing the MENU button on the control unit front panel, or by right-clicking with the mouse in the upper one-third of the display screen. The CAM menu is the initial menu displayed if none are present in the image display window, or it can be displayed from another menu by selecting the CAM tag. Using the panel buttons, menus can be changed by selecting (highlighting) the desired tag with the cursor buttons and pressing the MENU button to enter, or confirm, the selection. Pressing the MENU button again closes the menu screen. When using a mouse, menu screens are closed by right-clicking anywhere in the display outside the menu area.
One of the most important requirements in digital photomicrography is to provide the correct exposure to match the dynamic range of the specimen being imaged with the sensitivity and dynamic response of the camera system. The Digital Sight camera control unit is equipped with an exposure time control feature that allows the microscopist to capture precise and reproducible exposures of images observed in the microscope eyepieces or on the display monitor. 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.
Image exposure is a function of several variables, including the input brightness of the optical image originating from the microscope, the currently selected camera exposure mode, the exposure time, and compensation and gain settings of the camera system. The optimum exposure range, as determined by the Digital Sight metering system, is indicated by a green segmented exposure level indicator bar displayed in either of the CAM menus. The bar segments change to red if the exposure is excessive, and become blue to indicate underexposure. Figure 2 illustrates the various exposure level states that may be displayed by the indicator. Because the exposure level considered to be optimum varies with specimen type as well as the imaging mode employed, the level determined by the camera system should be regarded as a guide. An important feature of the Digital Sight 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. When any of the automatic exposure modes are utilized, if red or blue is displayed on the exposure level indicator, the illumination is outside the controllable range of the system, and adjustments to the microscope illumination are necessary to bring the exposure into the optimum range. Four basic exposure modes are provided by the camera control unit, which are described as follows:
Program Exposure Mode - In the PROG mode, the camera gain and the shutter speed are automatically varied in combination to yield the correct exposure, even if specimen brightness fluctuates or changes. The shutter speed is automatically adjusted over the range of 1/1000 second to 1 second, and the camera gain, or sensitivity, is automatically varied for additional exposure control. If the image appears excessively bright or dark, exposure compensation can be manually set within a range of -2.0 EV to +2.0 EV in 0.3-EV steps by adjustment of the CMP. icon.
Shutter Speed Priority AE Mode - Camera gain is adjusted automatically in S_AE mode to yield the correct exposure while maintaining the selected shutter speed. The fixed shutter speed can be adjusted over a range of 1/1000 second to 4 seconds. Exposure compensation can be applied to the automatically determined setting, in 0.3-EV steps, over a range of -2.0 EV to +2.0 EV.
Manual Exposure Mode - Selection of MANU mode permits the user to manually select the camera gain and shutter speed. The electronic shutter can be adjusted within the range of 1/1000 second to 60 seconds. Camera gain can also be adjusted over a range from x1 to x46, which is indicated on the display as C_gn. 100 to C_gn. 4600.
Focus Priority Exposure Mode - The F_AE mode is useful for imaging at low illumination levels, which causes difficulty in focusing due to the slow rate of image updates on the display monitor. Although the camera gain and shutter speed are automatically adjusted, the shutter speed is limited to a range that permits focusing, with additional exposure being accomplished by an increase in camera gain. The shutter speed changes automatically over a range of 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 within the range of -2.0 EV to +2.0 EV in 0.3 EV steps.
A bracketed series of exposures captured using the Digital Sight 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-third (1/3) second, while image (b) was exposed for one-seventh (1/7) second, and image (c) was exposed for one-fifteenth (1/15) second. The last three images (d-f) were exposed for one-thirtieth (1/30), one-sixtieth (1/60), and one-125th (1/125) 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 Digital Sight camera system's exposure controls, including exposure mode settings and other metering control functions, are accessible through several menu routes. Additional information on operating the system's exposure controls can be found in the following sections:
CAM Menu 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.
Room 1 Network Client Interface - Providing a network-based remote access to the Digital Sight'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 Digital Sight 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 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.
Contributing Authors
Matthew J. Parry-Hill, Thomas J. Fellers, and Michael W. Davidson - National High Magnetic Field Laboratory, 1800 East Paul Dirac Dr., The Florida State University, Tallahassee, Florida, 32310.
