Most of the patients under long application of anti-glaucoma eyedrops endure severe ocular surface irritation, which interrupt their quality of life a lot. Lots of studies aimed to search for new drugs for therapy. The investigators hypothesized that the artificial tears containing vitamin A or carbomer may be a great substitute. Both of the two drugs were in common use and had already been tested in animals.

Inclusion Criteria:

- diagnosed primary open-angle glaucoma or normal tension glaucoma patients with prostaglandin analogs treatment for more than one year.

Exclusion Criteria:

1. any systemic diseases which may cause ocular damage;
2. previous ocular trauma or surgery;
3. contact lens wear history in previous 6 months;
4. application of any artificial tears 3 months ago;
5. allergic to any of the drugs we used during examination.