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Dear colleagues:
Because of the differences in the way light travels through air and water, films taken of swimmers
above and below water need different scaling factors to convert 2D digitized coordinates to real
distances. I have been asked to determine center of mass velocity profiles of members of our men's
swim team at ASU. The pilot films I've been given were taken through an underwater window with
both above and underwater segments visible from a single camera. The image size is small and the
linear scale was only filmed below water (a mistake). I need to create an appropriate scaling factor for
the above water segments. I know some of you have encountered this problem before. I'd appreciate
hearing from some of you who have found a solution. Is there an general purpose correction factor
that can be applied in all cases? For example, can I just assume that images are approximately 33%
larger underwater than above water (a value based on some of Jim Hay's early work with inverse
periscopes)? Somehow I think not, given that different underwater windows or camera enclosures
involve different thicknesses of glass. Any help you could give would be appreciated. I'll post a
summary of replies.
Best regards,
--Rick
Richard N. Hinrichs, Ph.D.
Dept. of Exercise Science
Arizona State University
Hinrichs@asu.edu
Because of the differences in the way light travels through air and water, films taken of swimmers
above and below water need different scaling factors to convert 2D digitized coordinates to real
distances. I have been asked to determine center of mass velocity profiles of members of our men's
swim team at ASU. The pilot films I've been given were taken through an underwater window with
both above and underwater segments visible from a single camera. The image size is small and the
linear scale was only filmed below water (a mistake). I need to create an appropriate scaling factor for
the above water segments. I know some of you have encountered this problem before. I'd appreciate
hearing from some of you who have found a solution. Is there an general purpose correction factor
that can be applied in all cases? For example, can I just assume that images are approximately 33%
larger underwater than above water (a value based on some of Jim Hay's early work with inverse
periscopes)? Somehow I think not, given that different underwater windows or camera enclosures
involve different thicknesses of glass. Any help you could give would be appreciated. I'll post a
summary of replies.
Best regards,
--Rick
Richard N. Hinrichs, Ph.D.
Dept. of Exercise Science
Arizona State University
Hinrichs@asu.edu