Human eye The upper end of the range is given in terms of normal visual performance as 10 cd/m (100,000,000 or one hundred million candelas per square meter). The eye includes a lens similar to lenses found in optical instruments such as cameras and the same physics principles can be applied. The pupil of the human eye is its aperture; the iris is the diaphragm that serves as the aperture stop. Refraction in the cornea causes the effective aperture (the entrance pupil) to differ slightly from the physical pupil diameter. The entrance pupil is typically about 4 mm in diameter, although it
Diaphragm pacing an electric current through electrodes that are attached internally. The diaphragm contracts, expanding the chest cavity, causing air to be sucked into the lungs (inspiration). When not stimulated, the diaphragm relaxes and air moves out of the lungs (expiration). According to the United States Medicare system, phrenic nerve stimulators are indicated for "selected patients with partial or complete respiratory insufficiency" and "can be effective only if the patient has an intact phrenic nerve and diaphragm". Common patient diagnoses for phrenic nerve pacing include patients with spinal cord injury, central sleep apnea (i.e., Ondine's curse), and diaphragm paralysis. There are currently