We present the luminosity function and color-redshift relation of a magnitude-limited sample of 145 mostly red field E/S0 galaxies at z 1 from the DEEP Groth Strip Survey (GSS). Using nearby galaxy images as a training set, we develop a quantitative method to classify E/S0 galaxies based on smoothness, symmetry, and bulge-to-total light ratio. Using this method, we identify 145 E/S0's at 16.5 I 22 within the GSS, for which 44 spectroscopic redshifts (zspec) are available. Most of the galaxies with spectroscopic redshifts (86%) form a "red envelope" in the redshift-color diagram, consistent with predictions of spectral synthesis models in which the dominant stellar population is formed at redshifts z 1.5. We use the tight correlation between V-I and zspec for this red subset to estimate redshifts of the remaining E/S0's to an accuracy of ~10%, with the exception of a small number (16%) of blue interlopers at low redshift that are quantitatively classified as E/S0's but are not contained within the red envelope. Constructing a luminosity function of the full sample of 145 E/S0's, we find that there is about 1.1-1.9 mag brightening in rest-frame B-band luminosity back to z 0.8 from z = 0, consistent with other studies. Together with the red colors, this brightening is consistent with models in which the bulk of stars in red field E/S0's formed before zfor 1.5 and have been evolving rather quiescently, with few large starbursts since then. Evolution in the number density of field E/S0 galaxies is more difficult to measure, and uncertainties in the raw counts and their ratio to local samples might amount to as much as a factor of 2. Within that uncertainty, the number density of red E/S0's to z 0.8 seems relatively static, being comparable to or perhaps moderately less than that of local E/S0's, depending on the assumed cosmology. A doubling of E/S0 number density since z = 1 can be ruled out with high confidence (97%) if Ωm = 1. Taken together, our results are consistent with the hypothesis that the majority of luminous field E/S0's were already in place by z ~ 1, that the bulk of their stars were already fairly old, and that their number density has not changed by large amounts since then.
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