Apparatus comprising: a fluidic structure; and defined within the fluidic structure, a channel portion extending in a longitudinal direction and having, in each position along its length, a respective cross section; in each position's cross section, the channel portion having a boundary (90,94,122,124) along most of which it is surrounded by material with higher refractive index than a liquid; the fluidic structure comprising: a light-transmissive bounding component (184,200) configured to preserve a fluidic barrier between first and second portions of the channel; the fluidic structure further having defined therein at least two ports (150,152) through which a liquid can enter and exit the first portion; and a light-transmissive layer that extends in the longitudinal direction along at least part of the first portion's boundary and beyond the bounding component (184,200) to the second portion; wherein the layer and the bounding component (184,200) are positioned so that light can enter the second portion from the elongate component at a first angle of incidence with respect to the longitudinal direction, pass through the bounding component (184,200), and enter the first portion of the channel at a second angle of incidence with respect to the longitudinal direction, the second angle being smaller than the first angle; wherein the fluidic structure further includes an outer component in the second portion of the channel and outward from the bounding component (184,200), the second portion containing air, gas, or vacuum between the outer component and the bounding component (184,200). a fluidic structure; and defined within the fluidic structure, a channel portion extending in a longitudinal direction and having, in each position along its length, a respective cross section; in each position's cross section, the channel portion having a boundary (90,94,122,124) along most of which it is surrounded by material with higher refractive index than a liquid; the fluidic structure comprising: a light-transmissive bounding component (184,200) configured to preserve a fluidic barrier between first and second portions of the channel; the fluidic structure further having defined therein at least two ports (150,152) through which a liquid can enter and exit the first portion; and a light-transmissive layer that extends in the longitudinal direction along at least part of the first portion's boundary and beyond the bounding component (184,200) to the second portion; wherein the layer and the bounding component (184,200) are positioned so that light can enter the second portion from the elongate component at a first angle of incidence with respect to the longitudinal direction, pass through the bounding component (184,200), and enter the first portion of the channel at a second angle of incidence with respect to the longitudinal direction, the second angle being smaller than the first angle; wherein the fluidic structure further includes an outer component in the second portion of the channel and outward from the bounding component (184,200), the second portion containing air, gas, or vacuum between the outer component and the bounding component (184,200). a light-transmissive bounding component (184,200) configured to preserve a fluidic barrier between first and second portions of the channel; the fluidic structure further having defined therein at least two ports (150,152) through which a liquid can enter and exit the first portion; and a light-transmissive layer that extends in the longitudinal direction along at least part of the first portion's boundary and beyond the bounding component (184,200) to the second portion; wherein the layer and the bounding component (184,200) are positioned so that light can enter the second portion from the elongate component at a first angle of incidence with respect to the longitudinal direction, pass through the bounding component (184,200), and enter the first portion of the channel at a second angle of incidence with respect to the longitudinal direction, the second angle being smaller than the first angle; wherein the fluidic structure further includes an outer component in the second portion of the channel and outward from the bounding component (184,200), the second portion containing air, gas, or vacuum between the outer component and the bounding component (184,200).