You must have at least two things : first, something that can direct light from the source to the cell and secondly the cell (sensor) that can convert light into electricity.
The 1st part can be made very simply as a narrow tube which is long enough so the sensor can only receive light from the source.
Claude, that's an unusual set of definitions, you should consider calling W/sr "radiant intensity" to distinguish it from the notion of "intensity" that is normally used in at least early undergraduate physics.
We Value Quality
• Topics based on mainstream science
• Proper English grammar and spelling We Value Civility
• Positive and compassionate attitudes
• Patience while debating We Value Productivity
• Disciplined to remain on-topic
• Recognition of own weaknesses
• Solo and co-op problem solving