Designing with digital imagers -- either CCD or CMOS -- requires a full understanding of imager characteristics. One of the most-referenced characteristics of digital imagers is "Optical Format," often shortened to "OF". The sensor's OF is usually matched with the lens system and can have a dramatic effect on the system cost and performance. Optical format is simple in principle yet often misunderstood and misreported in practice.

First, the simple optical format equation: Take the diagonal dimension of the imaging area in millimeters and divide by 16. Express the result in inches.

Now, the curious part of the equation: Why mm? Why inches? Why 16?

Back in the "good-old days," imaging systems used vidicon tubes as the "image sensor." Vidicon tubes, similar to CRTs, were produced in standard sizes like 1 in., _ in., etc., but had a limited useable area. For example, even though a 1 in. Vidicon tube was 1 in. in diameter, it did NOT have 1 in. of good, visible image capture area. In fact, it offered only about 16 mm of useful area. Therefore, a lens manufacturer, making 1 in. optics only needed to provide about 16 mm of useable diameter.

Realizing the discrepancy between actual and useable area, a standard was set: 1 in. optics became 16 mm, _ in. became 8 mm, and so on. This standard from vacuum pick-up tubes was carried on until now because of the standardization of lens systems to sizes of 1 in., 2/3 in., _ in., etc. using the 16 mm rule. In addition, lens systems typically become lower cost with smaller format -- a _ in. lens is generally cheaper than a 1/3 in. lens, which is generally cheaper than a _ in. lens, and so on.

Because of the lens system cost model and the uncertainty of many engineers on how to calculate optical format directly, many imaging companies have started advertising their imagers with optical formats that don't exactly match.

For example, some manufacturers may bend the truth a bit on their optical format to fit within the lower-cost lens system. For example, a VGA imager (640 x 480 pixel resolution) with a 6-micron square pixel calculates to 0.30 in. optical format. While this fits between the _" and 1/3" optical format, the optimal solution is probably to use 1/3 in. optics. Using the smaller optical format will decrease lens cost but it may result in "vignetting," in which the corners of the image are dark since it is outside the lens capture area.

The final decision between using a _ in. and 1/3 in. system will be a cost/performance trade-off for the engineer, meaning that either system designers should calculate the exact optical format, or imager companies should start listing both the actual optical format in decimals as well as the "recommended" lens system to use with the imager.

Since most imager data sheets don't list actual optical format, but do list the size of their pixels in microns, a more helpful equation is to convert the pixel size, and array size, directly to optical format. The equation for this is:

Optical Format = ((sqrt(((w*p)^2)+((h*p)^2)))/16)/1000,

where,
w = width of array (number of pixels)
h = height of array (number of pixels)
p = pixel size (microns)

Using this formula for different sizes in VGA format you get the following:

Pixel Size (Microns)	VGA Optical Format (Inches)
5.0	0.25
5.6	0.28
6.0	0.30
6.6	0.33
7.0	0.35

For its own engineers, Y-Media has developed an application for the Mathpad program for Palm PDAs (Mathpad is available from www.palmgear.com for purchase or you can try it as shareware first at http://www.radiks.net/~rhuebner/). It can take the pixel size and array and give out optical format, or do the reverse and take an optical format and array size and output the correct pixel size. Readers interested in receiving the Y-Media optical format program can contact mitch@y-media.com for a free copy.

Authors:

Ed Jakl has been involved in solid-state imaging technology for over 18 ears, working at McDonnel-Douglas, Ford Aerospace, Rockwell Semiconductor, and is a co-founder of Y-Media. Previous imaging projects Mr. Jakl has worked on include missile guidance and space-based imaging systems, the Hubble Space Telescope, SOHO, Cassini, and the first digital camera flown on the Space Shuttle. Mr. Jakl was also the Principal Engineer for the imagers on the Mars Observer and the Mars Global Surveyor for which he won NASA Group Achievement Awards. Mr. Jakl earned a Bachelor of Science degree in Electrical Engineering with an emphasis on fiber optics and electro optics from the University of Arizona. He has authored articles and papers on CMOS imagers and CCD technology. He currently has 9 patents pending.

Mitch Reifel has been in the semiconductor industry for over eleven years working at Texas Instruments, Rockwell Semiconductor and Conexant Systems. Mr. Reifel's experience ranges from operations to product marketing, including management of TI's portfolio of floating point DSPs and Conexant's MFP products. Mr. Reifel is a co-founder of Y-Media. He earned his Bachelor of Science Degree in Electrical Engineering his MBA Rice University, and he is named as the co-inventor of two patent disclosures involving e-commerce business models.

Company Contact:
Y-Media, Inc.
5141 California Avenue, Suite 250
Irvine, CA 92612
949-784-1730

Analog Main | Product of the Week | Columns | Editorial | Tech Notes

		Click here to get your listing up.