BIG PIXELS VS. SMALL PIXELS THE OPTICAL BOTTLENECK Gregory Hollows Edmund Optics 1
IT ALL STARTS WITH THE SENSOR We have to begin with sensor technology to understand the road map Resolution will continue to increase Sensor speeds will continue to increase Applications split in to two areas Microns 12 10 8 6 4 2 Change in Resolution and Pixel Size 16 14 12 10 8 6 4 2 Resolution in Mega Pixels Pixel Size in Microns Total Sensor Resolution in MegaPixel Expon. (Pixel Size in Microns) Expon. (Total Sensor Resolution in MegaPixel) 0 0 1998 2003 2008 2013
SENSOR TECHNOLOGY DEVELOPMENT 29.0mp 25.0mp Increasing Resolution 14.0mp 10.0mp 5.0mp 5.0mp 0.3mp 0.3mp 2.0mp 0.3mp 1.3mp 1.3mp 1.3mp 12.0mp 9.0mp 8.0mp 6.0mp 8.0mp 4.0mp 4.0k 16.0mp 11.0mp 1.0mp 1/3 1/2 2/3 1 4/3 28mm dia. 43mm dia.
HOW DO WE DEFINE RESOLUTION? Resolution is a measurement of the imaging system's ability to reproduce object detail. Exaggerated example in which a pair of squares are not resolved(a) and resolved (b). In figure (a) the two squares are imaged onto neighboring pixels and are indistinguishable from one another. Without any space between, they appear as one large rectangle in the image. In order to distinguish them a certain amount of white space is needed, as in figure (b). This can be represented by a line pair.
HOW DO WE MEASURE RESOLUTION IN OPTICS? Frequency of lines is represented in line pairs over a linear spacing. Frequency or Line-pair(lp/mm) = Line-pair(lp) = 2 x Pixel
EXAMPLE: FIELD OF VIEW AND RESOLUTION 640 x 480 pixel (0.3 megapixels) 1600 x 1200 pixel (2 megapixels)
C-MOUNT SENSOR FORMATS 1 inch 9.6mm 1/3 inch 4.8mm 3.6mm 12.8mm 1/4 inch 2.4mm 3.2mm 2/3 inch 6.6mm 8.8mm Mega Pixel 15.15mm 1/2 inch 4.8mm 1.3 inch 6.4mm 15.15mm Common Area Sensor (4:3 Aspect Ratio) Common Name = Old Videcon Tube Diameter
IMAGE QUALITY Resolution Contrast
WHAT IS MEANT BY CONTRAST? Contrast describes the separation in intensity between blacks and whites. % Contrast = I max - I min I max + I min Reproducing object contrast is as important as reproducing object resolution. For an image to appear well defined black details need to appear black and the white details appear white. The greater the difference in intensity between a black and white line, the better the contrast.
HOW ARE CONTRAST AND RESOLUTION LINKED? Resolution and contrast are closely linked. Resolution is defined at a specific contrast. The typical limiting contrast of 10% is often used to define the limiting resolution level. For the human eye a contrast of 1-2% is often used to define resolution.
HOW DOES CONTRAST DEPEND ON FREQUENCY? Suppose two dots are placed close to each other and imaged through a lens. The two spots will blur slightly. Moving the spots closer causes the blur to overlap and contrast is decreased. When the spots are close enough that the contrast becomes limiting, the spacing is our resolution. At each spacing of the spots we obtain a specific contrast. We can plot this information in the form of a Modulation Transfer Function (MTF).
FREQUENCY AND MODULATION TRANSFER FUNCTION (MTF)
HOW DOES DIFFRACTION AND F/# AFFECT PERFORMANCE? Not even a perfectly designed and manufactured lens can accurately reproduce an object s detail and contrast. Diffraction will limit the performance of an ideal lens. The size of the aperture will affect the diffraction limit of a lens. The smallest achievable spot of a lens = 2.44 x wavelength of light x (F/#) F/# describes the light gathering ability of an imaging lens (lower F/# lenses collect more light). As lens aperture decreases, F/# increases.
HOW DOES DIFFRACTION AND F/# AFFECT PERFORMANCE? Not even a perfectly designed and manufactured lens can accurately reproduce an object s detail and contrast. Diffraction will limit the performance of an ideal lens. The size of the aperture will affect the diffraction limit of a lens. The smallest achievable spot of a lens = 2.44 x wavelength of light x (F/#) F/# describes the light gathering ability of an imaging lens (lower F/# lenses collect more light). As lens aperture decreases, F/# increases.
HOW DOES DIFFRACTION AND F/# AFFECT PERFORMANCE? The smallest achievable spot of a lens = 2.44 x wavelength of light x (F/#) 9 micron pixels 4.5 micron pixels ~f/8 ~f/4 ~f/2 2.2 micron pixels
WHAT IS A BETTER MTF? Depends on the application. Depends on the detector. Is limiting resolution important? Is high contrast at low frequencies important? 100 90 80 70 60 50 40 30 20 10 0 0 20 40 60 80 100
MODULATION TRANSFER FUNCTION (MTF) CURVE
ARE LENSES THE ONLY THINGS WITH MTF S? Each component of an imaging system has an MTF associated with it. Cameras, cables, monitor, capture boards, and eyes all have MTFs. Below is an example of the MTF of a typical CCD camera.
HOW DO INDIVIDUAL MTFS FORM A SYSTEM MTF? A rough estimate of system resolution can be found using the weakest link. This assumes that the system resolution will be determined by the lowest resolution of its components. A more accurate system resolution is one where the MTFs of each component are looked at and combined as a whole. Each component has its own MTF (Lens, camera, cables, capture board, and monitor). By multiplying each MTF we get a System MTF.
WHAT IS MEANT BY CONTRAST? Contrast describes the separation in intensity between blacks and whites. % Contrast = I max - I min I max + I min Reproducing object contrast is as important as reproducing object resolution. For an image to appear well defined black details need to appear black and the white details appear white. The greater the difference in intensity between a black and white line, the better the contrast.
HOW DOES DIFFRACTION AND F/# AFFECT PERFORMANCE? The smallest achievable spot of a lens = 2.44 x wavelength of light x (F/#) 9 micron pixels 4.5 micron pixels ~f/8 ~f/4 ~f/2 2.2 micron pixels
REAL WORLD LENS PERFORMANCE 5 MP 1/2.5 Inch 2.2 micron pixel 227lp/mm 113lp/mm 5 MP 2/3 Inch 3.45 micron pixel 145 lp/mm 72 lp/mm 4 MP 1.2 Inch 7.4 micron pixel 67 lp/mm 33 lp/mm Analysis on 50mm fl. at f/4, 300mm working distance Waveband is white light created by RGB LED s Assuming a 10% contrast noise floor for the sensors LP for each sensor are for 2 pixels per line pair and 4 pixels per line pair
HOW IS RESOLUTION TESTED? By imaging a test target, a limiting resolution can be found. Targets consist of varying frequencies. A common test target is the bar target. Bar targets have sets of line pairs. Orthogonal bars allow tests of astigmatic errors. Bar targets are limited by a finite number of steps in frequency.
REAL WORLD LENS PERFORMANCE 5 MP 1/2.5 Inch 2.2 micron pixel 113lp/mm Contrast: 30.3% 227lp/mm Contrast: 8.8%
REAL WORLD LENS PERFORMANCE 5 MP 2/3 Inch 3.45 micron pixel 72 lp/mm Contrast: 48% 145 lp/mm Contrast: 24.6%
REAL WORLD LENS PERFORMANCE 4 MP 1.2 Inch 7.4 micron pixel 33 lp/mm Contrast: 55.3% 67 lp/mm Contrast: 33.6%
REAL WORLD LENS PERFORMANCE 5 MP 1/2.5 Inch 5MP 2/3 Inch 4MP 1.2 Inch 2.2 micron pixel 3.45 micron pixel 7.4 micron pixel 30.3% Contrast 48.0% Contrast 55.3% Contrast 8.8% Contrast 24.6% Contrast 33.6% Contrast
HOW DOES DIFFRACTION AND F/# AFFECT PERFORMANCE? The smallest achievable spot of a lens = 2.44 x wavelength of light x (F/#)
COLOR (WAVELENGTH) MATTERS 3b 660nm Light 470nm Light
COLOR (WAVELENGTH) MATTERS
COLOR (WAVELENGTH) MATTERS
F2.8
F5.6
F8
YOU HAVE TO COMPROMISE BETWEEN RESOLUTION AND DOF
RESOLUTION AND DOF TRADEOFF
IT ALL STARTS WITH THE SENSOR We have to begin with sensor technology to understand the road map Resolution will continue to increase Sensor speeds will continue to increase Applications split in to two areas Microns 12 10 8 6 4 2 Change in Resolution and Pixel Size 16 14 12 10 8 6 4 2 Resolution in Mega Pixels Pixel Size in Microns Total Sensor Resolution in MegaPixel Expon. (Pixel Size in Microns) Expon. (Total Sensor Resolution in MegaPixel) 0 0 1998 2003 2008 2013 Year 37
FINALLY, DO YOUR HOMEWORK! Optics can process images at the speed of light. Give it the time it deserves! Specify what you need as a system not just components. Expect a lot from optical suppliers. They should know much more then just lens design.
Gregory Hollows Director, Imaging Business Unit Edmund Optics Barrington, New Jersey USA Phone: (856) 547-3488 Email: ghollows@edmundoptics.com www.edmundoptics.com