  We review the systematic uncertainties that have plagued attempts to obtain
high precision and high accuracy from ground-based photometric measurements
using CCDs. We identify two main challenges in breaking through the 1%
precision barrier: 1) fully characterizing atmospheric transmission, along the
instrument's line of sight, and 2) properly identifying, measuring and removing
instrumental artifacts. We discuss approximations and limitations inherent in
the present methodology, and we estimate their contributions to systematic
photometric uncertainties. We propose an alternative conceptual scheme for the
relative calibration of astronomical apparatus: the availability of calibrated
detectors whose relative spectral sensitivity is known to better than one part
in $10^3$ opens up the possibility of in situ relative throughput measurements,
normalized to a precision calibrated detector, using a stable but uncalibrated
narrowband light source. An implementation scheme is outlined, which exploits
the availability of tunable lasers to map out the relative wavelength response
of an imaging system, using a flatfield screen and a calibrated reference
photodiode. The merits and limitations of this scheme are discussed. In tandem
with careful measurements of atmospheric transmission, this approach could
potentially lead to reliable ground-based photometry with fractional
uncertainties below the percent level.
