We present new JHKL ^ { \prime } M ^ { \prime } photometry on the MKO system for a large sample of L and T dwarfs identified from SDSS and 2MASS and classified according to the scheme of [ ] geb02 .
We have compiled a sample of 105 L and T dwarfs that are uniformly classified and measured on a single photometric system .
The scattered JHK spectral indices and colors of L dwarfs are likely caused by variations in the altitudes , distributions , and thicknesses of condensate clouds .
Scatter in the H – K colors of late T dwarfs probably reflects the sensitivity of the K -band flux to pressure induced H _ { 2 } opacity , which itself is sensitive to surface gravity .
The M ^ { \prime } luminosities of late-T dwarfs are 1.5–2.5 times fainter than predicted under conditions of chemical equilibrium .
We have computed L _ { bol } and T _ { eff } for 42 L and T dwarfs whose trigonometric parallaxes have been measured .
We find that T _ { eff } \approx 1450 K for types L7–T4 , which supports recent models that attribute the changing JHK -band luminosities and spectral features across the L–T transition to rapid changes in the condensate clouds over a narrow range of T _ { eff } .
We compute T _ { eff } = 600 –750 K for 2MASS 0415-0935 ( T9 ) , which supplants Gl 570D as the coolest known brown dwarf .