An implantable article comprising: first and second parts (12,14); each of the first and second parts including a respective container (20, 22) and being operable as an optical cavity with a respective optical spectrum characteristic that can be affected by presence of spectrum-affecting objects in the container (20, 22); the respective container having a respective set of one or more bounding regions (30, 37) through which objects in bodily fluid can transfer between the container's interior and exterior when the article is implanted in a body; and first and second fluidic components that control transfer of objects in bodily fluid through the respective bounding regions (30, 37) of the first and second parts (12, 14), respectively; the first fluidic component permitting transfer of a first set of the spectrum-affecting objects into the first container at a more rapid rate than spectrum-affecting objects not in the first set; the second fluidic component permitting transfer of a second set of the spectrum-affecting objects into the second container at a more rapid rate than spectrum-affecting objects not in the second set, the first and second sets of spectrum-affecting objects both including a shared subset; characterized in that the first set including a non-shared subset that the second set does not include; objects that are instances of an analyte type being predominantly in the non-shared subset, and each part (12,14) including light-reflective components (310, 312) of respective optical cavities and a set of wall parts defining respective light-transmissive regions in respective containers (20, 22) between light-reflective components (310, 312), the article further comprising: a third part that is operable as a reference optical cavity with a respective optical spectrum characteristic; the third part having a respective container having no bounding regions (30, 37) through which objects in bodily fluid can transfer between the container's interior and exterior when the article is implanted in a body. first and second parts (12,14); each of the first and second parts including a respective container (20, 22) and being operable as an optical cavity with a respective optical spectrum characteristic that can be affected by presence of spectrum-affecting objects in the container (20, 22); the respective container having a respective set of one or more bounding regions (30, 37) through which objects in bodily fluid can transfer between the container's interior and exterior when the article is implanted in a body; and first and second fluidic components that control transfer of objects in bodily fluid through the respective bounding regions (30, 37) of the first and second parts (12, 14), respectively; the first fluidic component permitting transfer of a first set of the spectrum-affecting objects into the first container at a more rapid rate than spectrum-affecting objects not in the first set; the second fluidic component permitting transfer of a second set of the spectrum-affecting objects into the second container at a more rapid rate than spectrum-affecting objects not in the second set, the first and second sets of spectrum-affecting objects both including a shared subset; the first set including a non-shared subset that the second set does not include; objects that are instances of an analyte type being predominantly in the non-shared subset, and each part (12,14) including light-reflective components (310, 312) of respective optical cavities and a set of wall parts defining respective light-transmissive regions in respective containers (20, 22) between light-reflective components (310, 312), the article further comprising: a third part that is operable as a reference optical cavity with a respective optical spectrum characteristic; the third part having a respective container having no bounding regions (30, 37) through which objects in bodily fluid can transfer between the container's interior and exterior when the article is implanted in a body. a third part that is operable as a reference optical cavity with a respective optical spectrum characteristic; the third part having a respective container having no bounding regions (30, 37) through which objects in bodily fluid can transfer between the container's interior and exterior when the article is implanted in a body.