Worked example - fringe spacing in a hologram. Two beams of lambda = 633 nm (He-Ne) meet at angle theta = 30 degrees. Step 1: fringe spacing d = lambda / (2 sin(theta/2)). Step 2: sin(15 deg) = 0.259, so d = 633 / (2 x 0.259) = 633 / 0.518 = 1222 nm ~ 1.2 micrometres. Step 3: meaning - the plate must resolve ~800 lines/mm, and any vibration larger than a fraction of a micrometre during exposure smears the fringes. That is why holography tables float on air cushions in dark rooms.