The BIAS calibration frame is perhaps the easiest (and fastest!) to take. Consider taking a $0s$ integration time. What is the resulting signal?
$$L_{ij}(\lambda) = \Bigl[\epsilon_{ij}(\lambda)I_{ij}(\lambda) + T_{ij}(\lambda)\Bigr]t + B_{ij}(\lambda)$$
$$L_{ij}(\lambda) = \Bigl[\epsilon_{ij}(\lambda)I_{ij}(\lambda) + T_{ij}(\lambda)\Bigr]0 + B_{ij}(\lambda)$$
$$L_{ij}(\lambda) = B_{ij}(\lambda)$$
Yep. That's it.