solid line), $\sigma$ Orionis 71 (3-7 Myr, dashed line), and KPNO-Tau 4 (1-2 Myr, dotted line). The revised spectral types of these objects are indicated on the figure. The shaded areas are the wavelength regions over which the H$_{2}$(K) index is calculated.[]{data-label="fig:fig3"}](1Myr_sigma_field_smoothed_010813.pdf)

**H$_2$** Collision Induced Absorption in the K band
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It has previously been observed that the F$_\lambda$ spectra of young, late M-type brown dwarfs have a flat maximum between 2.18$\mu$m and 2.28$\mu$m [@luhman04a], while mature field dwarfs of the same spectral type have a peak between 2.14$\mu$m and 2.18$\mu$m then decline between 2.18$\mu$m and 2.29$\mu$m (L06). In other words, the K band spectra of younger objects appear redder than the K band spectra of older objects. This change in the K band spectrum is believed to be caused by increased collision-induced absorption (CIA) by H$_2$ in the atmosphere, owing to the increase in surface gravity with age ( @saumon12, hereafter S12). We wanted to see whether this behaviour was evident in our sample, and, if it was, to quantify it.

Measuring H$_2$ CIA
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We used the smoothed NIFS spectra of our brown dwarf calibrators to identify the locations of the peaks in their continua. Figure 3 demonstrates 