Sigma Research Inc. of Richland, Wash., said it has signed a letter of intent to acquire closely held Optical Instruments Corp. of Buena Park. Additional details, including the value of the agreement, should be released in 30 to 60 days when negotiations are completed, said Ted Warner, Sigma Research's manager of corporate development. Warner described the move as one which would "complement our current business activities." Sigma Research manufactures ultrasonic and optical testing systems used in industrial quality control and research and had profits of $381,000 on sales of $7.7 million last year. Warner and Eugene Thorburn, president of Optical Instruments, both said that no changes in location or staffing were planned for Optical Instruments, which has 63 employees and manufactures optical systems for use in military and industrial applications. Thorburn said that Optical Instruments, founded in 1964, had sales of about $2.8 million last year, and was projecting sales of about $3.2 million for the current fiscal year.