a. treating Spirulina biomass with acetone until chlorophyll a is completely extracted,

b. separating the extract by filtering or centrifuging the biomass,

c. treating the extract with acid in order to remove magnesium ion out of the chlorophyll molecule and to hydrolyse a phytyl ester group to form an acidic pheophytin a mixture

d. neutralizing the acidic pheophytin a mixture with a strong inorganic base, to precipitate pheophytin a which is then filtered,

e. hydrolyzing pheophytin a a mixture of hydrochloric acid-acetone-hexane with, 6-16 ml acetone, 0.6-6-ml hexane and 5-10 ml concentrated hydrochloric acid being used for every 1 g of crude pheophytin a,

f. heating the mixture up to 40-60° C. while stirring for a period of time from 20 minutes to 1 hour,

g. adding hexane (6-16 ml) to the mixture to form an organic phase and a water phase,

h. contacting the organic phase with a mixture of acetone and hydrochloric acid in a ratio from 2:1 to 10:1,

i. contacting the water phase that contains pheophorbide a with hexane,

j. then contacting the water phase containing pheophorbide a with an excess of sodium citrate (tri-, di- or mono-substituted) water solution to precipitate pheophorbide a,

k. filtering the precipitated pheophorbide a, washing the precipitate with water,

l. recrystallizing the precipitated pheophorbide a using an acetone-water mixture, and air drying until its weight becomes constant,

m. dissolving the pheophorbide a in acetone,

n. adding a strong inorganic base in the form of a water solution of 0.05-1.00% concentration, while stirring at 30-60° C. for 5-30 min,

o. adding additional strong inorganic base in the form of a water solution of 1-50% concentration to form a mixture,

p. heating the mixture of step o at 40-60° C. for 20-90 min,

q. neutralizing by adding diluted hydrochloric acid to precipitate chlorin e₆,

r. separating by centrifugation the chlorin e₆

s. washing the chlorin e₆ with distilled water until the acid reaction disappears,

t. recrystallizing the chlorin e₆55-80% with acetone to separate linear tetrapyrroles,

u. filtering the chlorin e₆ and washing with distilled water,

v. heating the chlorin e₆ in a sealed reservoir at a temperature of from 40-100° C. for a period of time from 1 hour to 30 days,

w. cooling the chlorin e₆ and adding a strong base solution until the pH is in the range from 7.5-8.5, and optionally gel-filtering the mixture,

x. adding diluted hydrochloric acid solution to the obtained solution until precipitation,

y. separating the precipitate by filtering or centrifugation, and

z. adding concentrated strong hydroxide solution until the pH is in the range from 7.0-8.5, to obtain the photosensitizer comprising 80-90% chlorin e₆, 2-15% purpurin 5 and purpurin 18-chlorin p₆.