High Dynamic Range (HDR) Overview
Why should you use HDR?
- To capture detail from a slide that exhibits too much intensity variation (or dynamic range) to be captured with a single exposure from the camera in use.
- To reduce noise.
In this case, a single exposure would leave parts of the slide underexposed (i.e., detail in dark regions is lost in camera noise) or overexposed (i.e., detail in bright regions is lost due to saturation). In contrast, HDR captures detail in all regions of the slide.
Noise reduction is greatest when the interval between exposures is minimized so that all regions of the slide are well-exposed in multiple images.
How does it work?
HDR combines the properly-exposed information from a series of images acquired with different exposures. You must select the exposure sequence manually (see Recommendations below).
Before you begin
- Because HDR combines information from multiple images, keep the specimen, stage and camera stationary and don't change the light.
- Calibrate the camera properly:
- Use a specimen exhibiting a range of intensities.
- Use exposures suitable for the observation of both underexposed and overexposed pixels.
- Use from 8 to 15 exposures.
- After calibration, only change exposure. Do not change any other camera parameter (e.g., camera gain, brightness).
Recommendations
A proper sequence of exposures is critical for a successful HDR acquisition.
- Select the shortest exposure so that there are no overexposed pixels.
TIP: Adjust the shutter speed until the maximum value on the camera histogram is at approximately 50%.
- Select the longest exposure so that you can see detail in the dimmest part of the slide.
- Select at least 3 to 5 exposures for the acquisition.
TIP: More exposures lead to better results when there is a wide dynamic range in the specimen.
Performing an HDR acquisition
- Calibrate the camera:
- Select Acquisition>HDR Calibration. The HDR Imaging dialog box appears.
- Enter values for the Exposure Setup.
- Use a specimen with a wide range of intensities (the specimen doesn't need to be representative of the specimens to be acquired).
- Exposure Range: Use exposures suitable for the observation of both underexposed and overexposed pixels.
- Lowest exposure value: Adjust the camera histogram so that the image is not saturated.
- Highest exposure value: Adjust the exposure so that you can see details in the darkest areas.
- Number of Exposures: 8 to 15 (greater than the number of exposures used for HDR Setup)
- Define the acquisition settings:
- Select Acquisition>HDR Setup for a single channel image, or Acquisition>Acquire Setup>Multichannel Acquires tab>HDR Setup button for each channel. The HDR Imaging dialog box appears.
- Enter values for the Exposure Setup.
- Exposure range:
- Lowest exposure value: Adjust the exposure until the maximum value on the camera histogram is at approximately 50% to avoid overexposed pixels.
- Highest exposure value: Adjust the exposure so that you can see details in the darkest areas.
- Number of exposures: 3 to 5 (increasing the number of exposures typically lead to better results, but the acquisition process takes longer)
About Exposure Setup - In the Acquisition menu, click Enable HDR to turn it on (a checkmark is added).
- Acquire the image (Acquisition>Acquire Image or Acquisition>Acquire Multichannel Image).
About Exposure Setup