The new Hologic ESPP boasts a 15% discount rate and a 6-month offering period with a look back provision.

FORBES: Employee Stock Purchase Plans Are Making A Comeback

Unlike non-statutory options, the offering period for incentive stock options is always 10 years, after which time the options expire.

FORBES: Introduction To Incentive Stock Options

The Plan then uses the employees money that has been payroll deducted during the offering period to automatically purchase shares at a 15% discount to the purchase price.

FORBES: $1.6 Million For Apple Employees, Up $500,000 In Six Months