  Mean radiant temperature (MRT) is the primary metric of radiant heat exchange
between a human body and the environment, and it dominates human thermal
comfort and heat stress. This study develops a new portable recording system
for spherical thermography utilizing only commercial devices (an infrared
thermal imaging camera and a portable rotation platform) and image processing
for panorama synthesis. We use the system to estimate the MRT in a longwave
radiation environment. A spherical thermal image is generated by synthesizing
24 source images on the basis of feature point identification. The longwave MRT
and plane radiant temperature can be estimated from the spherical thermal image
using image projection methods, including the orthographic projection and
Lambert cylindrical projection. To validate the developed system, measurements
were made using the system in outdoor and indoor environments with various
radiant temperature distributions, including sunny built spaces and a
tree-shaded space, and the results were compared with those obtained using
pyrgeometers. The difference in longwave MRT between the estimation by the
developed system with orthographic projection of spherical thermal images and
the measurement by pyrgeometers for six directions was within 1 {\deg}C in most
cases and 1.6 {\deg}C at maximum. The results show that the developed system
has sufficient accuracy for longwave MRT estimation while evaluating the
radiant temperature distribution and radiant asymmetry of spaces.
