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Basics of Digital ImagingDigital Image Capture and Display
Kevin L. Lorick, Ph.D.FDA, CDRH, OIVD, DIHD
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Digital Imaging
In the context of today’s discussion, whole slide imaging is the creation of digital images
from glass slides.
The creation of these images depends upon computer technologies whose purpose is to
convey to the digital image the same information contained on the glass slides.
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Digital Imaging• Collection Of Images On A Pixel-Based
Collector. (CCD, CMOS, PMT)
• Pixel=Picture element– Spatial Area (Shape, and size described in ppi)– Issues Of Color (bits/pixel) 1 bit color=pure B&W.
24 bit color =16.8 million colors.– Sensitivity To Photons (important at higher
magnifications)
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Image Resolution and Digital Capture
• In order to digitize a microscope specimen without losing any details available to the human eye, the dimension of the detector element (e.g. CCD pixels) should be no larger than one half the corresponding limiting resolution distance.
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Image Resolution• The limiting resolution for a particular lens is determined
by its numerical aperture.• For example, a 20X lens requires a numerical aperture
no greater than 0.5 NA.• The limiting resolution of this lens is approximately 0.8
uM (31250 resolution distances/inch x 2 pixels=62500 ppi)
• A camera detector would require this pixel density to avoid losing resolution OR require additional processing
• A high end 21.1 megapixel consumer camera contains 3873 ppi (1/16th of the information in one dimension).
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Other Digital Color Image Approaches
Beam Splitter
Rotating Filter
Scanning Mask Or Filter
Monochrome Pixel Presented
Color Sequentially
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Black And White
• Different Wavelengths Of Light Appear Identical In Luminance When Converted From HSV To Black & White
• B&W Is 10 Logs Of Intensity• Each pixel may record the luminosity of each color in
terms of bits (binary digits)
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Bit Depth Number Of Combinations Per Pixel
• 1 bit-(0,1) (On Or Off)
• 2 bit-4 (00, 01,10,11)
• 3 bit-8 (000,001,010,100,011,101,110,111)
• 4 bit-16 Old B&W• 5 bit-32• 6 bit-64 Common B&W, Old Color
• 7 bit-128• 8 bit-256 Old Color
• 9 bit-512• 10 bit-1024• 11 bit-2048• 12 bit-4096• 13 bit-8192• 14 bit-16,384 Current Std
• 15 bit-32,768• 16 bit-65,536• 24 bit-16 million-”true color”
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Image AcquistionSingle Or Composite
Publication Or Analysis
Magnification
Format
File Type
TIFF, BMP, PNG, JPEG, etc.
Color Or B&W
Single Or Multiple
Resolution
Image Processing
Camera Settings
Post-Image Processing
-Compression
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• Resolution• White Correction
– Color Temperature
• Black Correction– In Combination With White Correction,
Defines Dynamic Range
• Background Correction– Means Of Correcting Aberrations From
Illumination & Other Imperfections
Camera Calibration
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Challenges In Calibration
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1 0
2 0
3 0
4 0
5 0
6 0
7 0
8 0
9 0
1 00
0 1 2 3 4 5 6 7 8 9 1 0
Intensity
Intensity
Sin
gle
Poin
t
Over Extended
Min
- Max
10 -
90
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Image Capture-Storage
• A typical 1.4 megapixel image requires
1.4 MB of storage space (8 bit color).
• A 21.1 megapixel truecolor (24 bit) image requires 63.3 MBs.
• To achieve “ideal” resolution a single image would require >16 GB to capture a single field of view.
• Alternatives may be time consuming.
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Field Of View• What The Camera
Images Is A Central Region Of The Field Of View In The Ocular– Frequently Off
Center• Results In Shading
Of The ImageField Of View,
Through The Ocular
400 X Magnification
Field Of View,
Captured By A Camera
100 X Magnification
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Digital Imaging Techniques
Point Mapping
Detector type
Line Mapping
Global Imaging
Single -point
Linear array
Focal-plane array
Image Acquisition
Technique Tradeoffs
Very Slow Image acquisition
Issues Of Spatial Resolution
Slow Image acquisition
Better Spatial Resolution
Fast Image acquisition
Highest Potential Spatial Resolution
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232 x232
464 x464
1360 x 1360
2060 x 2060
3090 x 3090
200 pixels
200
pixe
ls
Image SizeImage Display
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Image Formats• Uncompressed
– Each Pixel Recorded With Full Color Depth– Large Files
• Lossless Compression– Color Values Of Pixels Unaltered
• Lossy Compression (typically Fourier Transform)– Reduction in Spatial Detail– Reduction In Number Of Colors Recorded
• Removal Of Colors That Are Rare/Not Present In Image
• Decreased Bit-Depth Of Color
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Image display
-It is possible to capture an image with one pixel arrangement and display with another.
e.g. Monitor typically has 72 ppi and displays 1680 x 1050 pixel (3.2:2 ratio w/h)
The 21.1 MP camera captures 3873 ppi on a 36 x 24 (3.0:2 ratio).