J. Francis, "Improved Test Methods for Elliptical and Spherical TCT Mirrors,", ATM J. 14, 34-39, (ISSN 1074-2697).
Unfortunately, references to the LWT on the Internet have mostly faded away. Jim Burrows author of Sixtests.exe does mention the LWT in passing at http://www.raddobs.com/atm/atm_math.htm.
LWT was a variant of the moving source Caustic where instead of trying to determine a Caustic zone holes focus laterally and longitudinally precisely, the LWT measurements were done manually in the same lateral plane. Being that for the LWT the
longitudinal value was irrelevant, didn't require a mask and little judgment was required to determine laterally where a zone ray crossed the lateral plane, the LWT was an improvement over the standard Caustic. However, the required lateral
accuracy required was very tight. Given small room temperature and the time to take the measurements, I could not get repeatable LWT results.