Test points (45L,5U); (45L,H); (45L,5D): Min 45cd total for group
Test points (45R,5U); (45R,H); (45R,5D): Min 45 cd total for group
Test points (30L,H); (30L,5D): Min 50cd total for group
Test points (30R,H); (30R,5D): Min 50cd total for group
Test points (10L,10U); (10L,5U); (V,10U); (V,5U); (10R,10U); (10R,5U): Min 100cd total for group
Test points (10L,H); (10L,5D); (V,H); (V,5D); (10R,H); (10R,5D): Min 360cd total for group
When 2 lamps of the same or symmetrically opposite design are used, the reading along the vertical axis and the averages of the readings for the same angles left and right of vertical for 1 lamp shall be used to determine compliance with the requirements. If 2 lamps of differing designs are used, they shall be tested individually and the values added to determine that the combined units meet twice the candela requirements. When only 1 backup lamp is used on the vehicle, it shall be tested to twice the candela requirements.
< end cite from FMVSS 108>
I don't have a copy of J593c/February 1968, but I do have the September 2005 version which has the same zone groups and zone minimum requirements, and also specifies minimum candela values at each test point as follows:
(45L & 45R, 5U & H & 5D): 15 cd at each point
(30L & 30R, H & 5D): 25 cd at each point
(10L & 10R, 10U): 10 cd at each point
(10L & 10R, 5U): 20 cd at each point
(H & 5D, 10L & 10R): 50 cd at each point
(10U, V): 15 cd
(5U, V): 25 cd
(H, V): 80 cd
(5D, V): 80 cd
It also specifies that when using the zone group requirements, no single test point shall be less than 60% of the individual test point minimum.
It also specifies maximum anywhere at H and above:
300cd for a lamp used as part of a 2-lamp system
500cd for a lamp used singly
I don't know that these provisions are present in the earlier version of J593, but I think they probably are. That maximum above horizontal is a glare-prevention measure, and it's a sensible one. You're going to turn on white lights at night; you don't want them shining uncontrolled amounts of light in other drivers' eyes. I can think of a lot of fog lamps that would comply with the spirit and often with the specs of the J593/FMVSS108 requirements for backup lamps, and would give you a lot more light than a vehicle's ordinary factory-equipment backup lamps.
Fed regs dont mean squat. state inspection is all you gotta worry about, and i went with a similar setup for 3 yrs, never got turned down for inspection or otherwise bothered. i HAVE thought about mounting some inconspicuous lights somewhere in the rear to blind tailgaters tho